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STR testing is performed on DNA isolated from forensic case samples for a variety of reasons. Some of the common reasons are: (1) to increase the chance of excluding a falsely-accused individual, (2) to aid in determining whether a sample contains a mixture of DNA from more than one individual, (3) to aid in the interpretation of data from a sample containing a mixture of DNA, and (4) to further limit the number of individuals included as a possible donor to the DNA obtained from a sample by providing increased statistical frequencies. Examples of some of the advantages of using STR testing in three cases are presented below. For (1), see case 1 below; for (2) and (3), see case 2 below; and for (4), see case 3 below.

Table 1. DQA1 Results in Post-conviction Case

Table 2. CTT-A Results in a Post-conviction Case

The results from the root of hair 1 gave no indication of a mixed sample with either CTT-A or PM testing (data not shown) and indicate that the DNA obtained from the root of hair 1 is from a male. The secondary types obtained from the root of hair 2 are consistent with the types from the root of hair 1 suggesting that the two hairs could have come from the same male. The defendant is excluded as the source of the DNA obtained from the root of hair 1 and also is excluded as a source of the DNA obtained from the root and the shaft of hair 2. The results are consistent with the primary types obtained from the "root" of hair 2 being the same DNA that was deposited on the shaft of hair 2 and are consistent with the types obtained from the victim. This illustrates the importance of doing shaft controls when hair roots are tested. In this case, the CTT-A results were crucial for demonstrating that: (1) the hairs could be associated with the victim based on the results on the shaft since the pubic hairs were collected two weeks after the crime and after the victim had moved out of the apartment, and (2) the pubic hairs from a male did not come from the defendant. No testimony of STR and PM results were given in this case.

Based on the reporting of the STR and PM results, the state dropped the charges against the defendant and he was released from prison.

In this case, a female was found dead with a towel wrapped around her bloody head and blood was found under her fingernails. A rock with hairs and tissue attached was found nearby, and the suspect, who had been seen with a similar towel earlier, was found to have a scratch on his leg consistent with the position of the rock containing tissue. DQA1, PM and CTT-A testing was performed on these samples and the results are in Tables 3 & 4.

The results of the DQA1, PM and CTT-A tests are consistent with the DNA obtained from the tissue on the rock being from a single male donor; the results are consistent with the types obtained from the suspect. The results from the fingernails and the towel indicate that DNA was obtained from at least two sources, and that one source is female and one source is male. Furthermore, the results from the two mixed samples showed distinct intensity differences for many of the loci. These intensity differences were used [as recommended by The National Research Council Committee on DNA Forensic Science (1)] along with the homozygous results obtained at several of the loci to interpret the data. The victim cannot be excluded as the primary source of the DNA obtained from the fingernails and the towel, and the suspect cannot be excluded as the secondary source of the DNA. With the assumption that there are only two sources of DNA in these two samples, it is possible to determine the types of the primary donor of the DNA as well as determine the types for all of the possible combinations of secondary DNA sources.

There was a challenge to the PCR results since there was no appellate decision for any type of PCR testing in the state and an admissibility hearing was held. Information regarding the RFLP and PCR technologies and their uses in forensic laboratories was presented to the court. The use of PCR and STRs in other fields, the validation work done by Cellmark Diagnostics and by other laboratories, and proficiency testing were also discussed at the pre-trial hearing. The court allowed the STR results to be presented to the jury at trial in a chart format similar to that presented above. Statistical frequencies for the types obtained from the tissue on the rock and statistical frequencies for the possible secondary types obtained from the fingernails and the towel were also presented. Similar statistical calculations for samples containing mixtures of DNA had been presented in an admissibility hearing and to a jury in the case of The State of New Jersey v. Nathaniel Harvey; the PCR results and statistical frequencies were accepted by the state appellate court [151 N.J. 117, 699 A.2d 306 (1997)].

Case 3

The Commonwealth of Massachusetts at that time had appellate rulings regarding RFLP testing but there were no appellate rulings for PCR testing. An admissibility hearing was held in this case in which testimony was given regarding the technologies involved in RFLP and PCR testing, the types of genetic polymorphisms recognized with each test system, and the validation studies conducted for each test system along with other information regarding the validity of human DNA identification testing in forensics and other fields. In addition, testimony was provided regarding PCR databases, review of the databases by independent experts, and the advantages of STR testing such as the existence of a limited number of discrete alleles observed in human populations. The PCR test results were admitted at trial and presented to the jury. This case, along with several other cases, was appealed to the Supreme Judicial Court of Massachusetts. In decisions resulting from two of these cases, the appellate court accepted PCR testing [Commonwealth of Massachusetts v. Vao Sok; 425 Mass. 787, 683 N.E. 2d 671 (1997)] and STR testing [Commonwealth of Massachusetts v. Adam Rosier; 425 Mass. 807, 685 N.E. 2d 739 (1997)]. To our knowledge, this is the first case in the United States where STR testing results have been reviewed by an appellate court.

Cellmark analysts have been to court in over 30 cases where CTT and/or CTT-A data have been presented to the trier of fact. Testimony was given in admissibility hearings prior to the trial for some of the cases. As in other admissibility hearings for DQA1, PM, D1S80 and RFLP testing, the testimony presented generally included information regarding the wide use of PCR and STR testing in other fields, the genetic bases for the polymorphisms, a description of the technology and types of results, validation studies including relevant publications, training and experience of the scientist and the laboratory, proficiency testing, controls performed, and safeguards in evidence handling and testing to ensure accurate and reliable results. Presentations at trial have ranged from a brief description of the technology and a summary of the data to more extensive testimony including areas routinely covered in admissibility hearings and test results discussed in detail.

The issues raised in cross-examination have generally been similar to those raised previously for other types of PCR testing and for RFLP testing. Such issues include whether STR testing is "new and novel", does multiplexing compromise the assay, is PCR testing so sensitive that contamination may invalidate the results, and are small databases representative of larger populations. Each of these issues can be addressed by the expert witness through publications of validation studies and databases, the use of appropriate laboratory standard operating procedures for evidence handling, testing, and use of controls, through proper training of the laboratory staff and the use of proficiency tests, and through the application of relevant guidelines such as those from TWGDAM (2) and the DNA Advisory Board.

STR testing results may provide useful information in many types of cases where DNA testing is possible. Since the genetic analysis of STR sequences in DNA has been widely used and accepted by molecular biologists in many areas of study for over 7 years, the use of STRs is not considered to be a new technique to the scientific community. We, as scientists, can be confident taking the results of STR testing into court as long as the data are supported by good laboratory practices. Our role as scientific expert witnesses is: (1) to be prepared with the appropriate validation studies, training, standard operating procedures including the use of appropriate controls, proficiency testing, etc. to support the STR data, (2) to work closely with the attorney or other relevant individuals to determine which cases can benefit from the use of STR testing, and (3) to fairly and accurately present those data in court to the trier of fact.

I would like to acknowledge Anjali Swienton, Melisa Weber and Paula Yates for their diligence and expertise in processing the samples and analyzing the data in the three cases presented; Dr. Robin Cotton for her help and leadership in bringing STR testing into our laboratory and assistance in reviewing data along with Dr. Jennifer Reynolds and Dr. Lisa Forman; and Jodi Kriss and David Sipes for their excellent work on the validation studies on CTT and CTT-A at Cellmark Diagnostics.

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