Rockne P. Harmon
Alameda County District Attorney’s Office

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How much impact will the addition of STR multiplexes have on the ability of prosecutors to hold criminals accountable for their actions? From a prosecutor’s perspective, there are a number of factors to consider in answering this question. Those factors are processing time, local capability, discrimination power, interpretation simplicity, and sample size/preservation. The following discussion will demonstrate why the availability of multiplex STR systems should satisfy the needs of prosecutors in almost all cases in seeing that guilty individuals are held accountable for their crimes.

Quite often there may be legally mandated time limits within which testing must be completed or the charges dismissed. Typically, the shortest of those time limits is ten working days. STR multiplexing may easily be competed within most legally mandated time limits.

Having a local capability allows for the setting of priorities within the community, taking all cases into consideration. This is a no-win situation, but it is possible with the local capability, not possible when dealing with private labs or distant government entities.

The availability of results, which may either uniquely identify or merely corroborate, from a local lab should be the ultimate goal.

The ability to resolve mixtures is important in many cases. Some of these cases may be sexual assault cases involving two assailants, or with a prior consensual sex partner.

In some jurisdictions there may be legal consequences if all the sample is consumed by prosecution testing. In addition, if sample is preserved for defense testing, and the defense elects not to test the evidence, this provides powerful argument for the prosecutor at the close of the trial. The availability of STR multiplexes helps address both of these issues due to minimal sample consumption.

Except for rare instances, cases utilizing forensic DNA typing can be grouped into standard cases, special cases, and offender database applications. The following discussion will demonstrate that the current availability of PCR-STR multiplex systems will satisfy the needs of the legal system in ensuring that guilty offenders will be held accountable for their crimes.

These cases may run the gamut from cases in which there is ample DNA for any and all forms of testing to those cases in which the sample is old or small. In these cases the testing may be conducted before criminal charges are filed, where there is no legal clock ticking requiring expeditious testing. At the other end of the spectrum, if charges have already been filed, there may be a short period of time, such as ten or thirty days, within which the tests must be completed or the case dismissed. In these standard cases, DNA may be the predominant form of evidence, or it may simply corroborate the victim’s identification of the defendant or the defendant’s confession.

STR multiplexes can satisfy all of these requirements, providing speedy, powerful association evidence or mere corroboration.

In some criminal cases, the hypothesis being tested is more limited and clearly defined than in others. In these cases, the availability of STR multiplexes can resolve important questions. Assume the following scenario to appreciate this power. A suspect is identified soon after a homicide wearing a T-shirt with many very small red dots, soon after the brutal murders of a father and three of his daughters. He provides a statement explaining his connection to the crime scene, but denying his responsibility for these killings.

STR multiplex typing on a large number of these tiny stains told a story quite at odds with the suspect’s story and lead to a guilty plea. (The work on this case was done by Susan Ballou from the Montgomery County Police Department Lab and by Cellmark Diagnostics.)

Almost every state in the United States now has an offender DNA database statute, requiring convicted offenders to provide samples. In the near future the states will be connected nationally by CODIS. Most states began typing the offender samples using RFLP. Those databases will ultimately be retyped using STR typing. Unsolved or unsolvable cases searched against the database may be either new suspectless cases or old suspectless cases. As part of Alameda County’s suspectless case submission program, approximately 1/3 of the suspectless cases processed are not suitable for RFLP typing. There are a number of different reasons for this. For the most part, the PCR-only cases are the older cases. In these cases, the samples are either too small, have been improperly stored, or have been partially consumed below the RFLP threshold in earlier attempts to solve the cases. A number of these cases pre-date the use of DNA typing.

Unanticipated mixtures may be encountered in these cases. A number of them represent apparent serial homicides where the victim has some history of prostitution. Biological evidence derived from vaginal, rectal, or oral swabs exists. What is unknown is whether or not the sperm is from a customer, the killer, or from both. If it is a mixture, STR typing may be able to help resolve the mixture and allow the STR profile to be successfully searched against the database.

The current availability of a number of different STR multiplexes now provides the opportunity to be in an ideal situation where there will be a local capability to establish a unique association in a timely manner. Meaningful results will be obtained from limited samples, in standard cases, special cases, and offender database search cases.