Day 70 was consumed by direct examination of DNA expert Dr. Robin Cotton, who methodically educated the jury on PCR and RFLP testing methodologies, Cellmark Diagnostics' laboratory procedures, and chain-of-custody protocols. Underlying this technical testimony, however, simmered strategic tensions: the defense objected to Cotton's qualification to testify about population statistics, the prosecution sought to present DNA mixture results without statistical frequencies, and the court heard dueling arguments on whether the jury needed special instructions to avoid conflating laboratory error rates with population probabilities.

• Dr. Robin Cotton's direct examination covered PCR methodology, genetic markers (DQ-alpha, five poly-marker loci, d1s80), contamination prevention procedures, and Cellmark's accreditation and quality controls.
• Defense attorney Peter Neufeld objected at sidebar that Cotton, qualified only as a microbiologist, lacked expertise in population genetics and statistics required to testify about match probabilities; Judge Ito overruled.
• Prosecution and defense clashed over whether DNA mixture results could be presented without statistical frequencies, with Neufeld citing People v. Barney and the NRC report as requiring statistical context.
• Cellmark disclosed two historical proficiency test errors (1988, 1989) with incorrect sample matches, along with procedural remedies implemented in response.
• The court-ordered July 28, 1994 visit by defense expert Dr. Blake to cut 10 percent samples from six evidence items was repeatedly discussed; the court clarified Blake made cuts but left all samples at Cellmark.
• RFLP autoradiographs showing blind-tested DNA results for Simpson, Nicole Brown, Ronald Goldman, and item 56 (shoeprint) were introduced for jury distribution.
• A jury instruction hearing concluded the day, with Barry Scheck arguing for clarifying language on population frequencies versus laboratory error rates, and Judge Ito deferring a ruling pending written submissions by Monday.