The previous ovservation that in dividing sister cells of Schizosaccharomyces pombe only one of two parallel divisions can be accompanied by a switch of mating type, herein termed "Miyata's rule", has been confirmed in pedigrees of diploid cells heterozygous for the mat2-Pm-B102 allele. Moreover, this rule appears to operate at the level of individual chromosomes, since in diploid cells simultaneous single-switch events were frequently observed in both sister cells, albeit on different chromosomes. Assuming two successive precursory states for the smt switching signal to the right of mat1, a deterministic 3-step model coupled to the cell cycle has been fitted to the empirical frequency distribution of conjugation in "four-lined cells" (a minipedigree of dividing sister cells). The nature of the first intermediate is still unknown, while the ultimate precursor of a switching event is probably a double-strand cut at smt, which can be revealed by molecular analyses. 