James M. Robertson¹, P. Sean Walsh² and Bruce Budowle^1
1 Forensic Science Research and Training Center, FBI Academy, Quantico, VA
² Applied Biosystems Division, Perkin-Elmer Corporation, Foster City, CA

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The CTT system, composed of the STR loci CSF1PO, TPOX and TH01, is being validated for forensic casework and population databases in many laboratories in the U.S.A. There have been reports of spurious bands and unbalanced signals across the loci. Using re-designed primer sequences, a higher specificity at low T_anneal has been reached, relative to the results obtained with the original primers. The T_ms have been matched to make the signals more uniform. Primers for the amelogenin gene, present on both the X- and Y-chromosomes, have been added to the CTT system to provide a test of gender (i.e. "CTTA"). The complete system works well in the 1.0-1.5 mM range of Mg²⁺ ion concentration, thus providing a window for variation. The new CTTA system has been optimized in kit form, complete with all necessary reagents so that only template need be added by the analyst.

Another STR system composed of the polymorphisms at D5S818, D13S317 and D7S820 (described at the Sixth International Symposium on Human Identification) has been validated by R. Fourney and colleagues (RCMP, Ottawa) and could be combined with the CTTA system for a greater overall power of discrimination. Since CSF1PO and D5S818 are both located on chromosome 5, we undertook an analysis of linkage of the two loci for genotypes obtained from a Caucasian database.