DNA analyst Renee Montgomery testified for the prosecution about D1S80 blood typing results from crime scene evidence. The defense's aggressive cross-examination exposed critical weaknesses in both Montgomery's qualifications and the system itself: she had never performed D1S80 testing before developing the program for this case, the system lacked proper validation and prior casework experience, and multiple technical anomalies plagued the evidence gels. Most damaging to the prosecution, Montgomery conceded that Simpson is excluded from all 23 stains tested on the victims' clothing.

• Renee Montgomery qualified as DNA expert and began direct testimony on D1S80 marker system
• Montgomery admitted she had never performed D1S80 testing before being assigned to develop the program for this case
• Defense established D1S80 had limited prior casework use, only voluntary guidelines, and internally-prepared proficiency tests
• Montgomery conceded Simpson is excluded as a contributor from all 23 stains tested on the victims' clothing
• Prosecution presented D1S80 results from Bronco and Rockingham glove showing mixtures and single-source matches
• Defense exposed multiple technical anomalies: gel distortions, shadow bands in reference lanes, control band intensity discrepancies
• Montgomery testified alleles linking Simpson to evidence samples were present in extremely small quantities (around 1 nanogram)
• Defense challenged whether scanned/magnified gel images revealed anomalous band-like activity suggesting a potential 17 allele
• Multiple sidebars arose over demonstrative slides and computer-enhanced photographs used in cross-examination