We have devised a PCR-based sexing method that is quick, simple, and highly reproducible. DNA is first extracted from embryonic mouse yolk sac via a 15 min, two-step incubation procedure utilizing PCR-compatible proteinase K buffer. Without any further manipulation the lysate is subjected to 30 cycles of PCR, optimized to run in less than 1 hr. The reaction includes multiplexed primer pairs for Sry and Myog (myogenin) that generate a male specific band of 441 bp and an internal control band of 245 bp, respectively. This robust method is used routinely in our laboratory and gives rapid genotyping results with 98% reliability and 100% accuracy.