The fission yeast Schizosaccharomyces pombe is able to switch sex - from M to P mating-type and back again. Two generations of asymmetric cell division are required to allow one of the four 'granddaughter' cells to switch. Arcangioli describes in the 15th August issue of EMBO Reports (EMBO Reports 2000, 1:145-150) the use of classic density gradient centrifugation techniques developed by Meselson-Stahl forty years ago (Proc Natl Acad Sci 1958, 44:671-682) to follow the fate of DNA strands at the mating-type locus during mitotic division. The study shows that one quarter of the DNA at the mating locus results from de novo synthesis of both strands, supporting a gene-conversion model in which a site- and strand-specific DNA break marks one of the sister chromatids, leading to asymmetry in the chromatids inherited by the two daughters and allowing cells to change sex at the subsequent mitosis.