Internal validation of STRmix™ – A multi laboratory response to PCAST BrightJo-Anne RichardsRebecca KruijverMaarten KellyHannah McGovernCatherine MageeAlan McWhorterAndrew CieckoAnne PeckBrian BaumgartnerChase BuettnerChristina McWilliamsScott McKennaClaire GallacherColin MallinderBen WrightDarren JohnsonDeven CatellaDorothy LienEugene O’ConnorCraig DuncanGeorge BundyJason EchardJillian LoweJohn StewartJoshua CorradoKathleen GentileSheila KaplanMarla HasslerMichelle McDonaldNaomi HulmePaul OefeleinRachel H. MontpetitShawn StrongMelissa NoëlSarah MalsomSimon MyersSteven WeltiSusan MorettiTamyra McMahonTeresa GrillThomas KalafutTim Greer-RitzheimerMaryMargaret BeamerVickie TaylorDuncan BuckletonJohn S. 2020 We report a large compilation of the internal validations of the probabilistic genotyping software STRmix™. Thirty one laboratories contributed data resulting in 2825 mixtures comprising three to six donors and a wide range of multiplex, equipment, mixture proportions and templates. Previously reported trends in the LR were confirmed including less discriminatory LRs occurring both for donors and non-donors at low template (for the donor in question) and at high contributor number. We were unable to isolate an effect of allelic sharing. Any apparent effect appears to be largely confounded with increased contributor number.