AbstractWe have previously reported that the HMRa‐bearing restriction fragment of a ρ° sir4‐11 strain (HMLα‐MATα‐HMRa), which acts as an α‐mater because of being ρ°, changes its electrophoretic mobility when the strain mates with a certain group of a‐mating strains (HMLα‐MATa‐HMRa). In this study, we found that the sir4‐11 strain being ρ° was not essential for this phenomenon and also that the altered form of the fragment contained HMRα in place of HMRa. Furthermore, we observed conversion of HMLa to HMLα in the cross in which a sir4‐11 HMLa‐MATα‐HMRα strain was mated with a representative of the above‐mentioned a‐mating strain. In addition, when this a‐mating strain was mated with a SIR+ HMLα‐MATa‐HMRα strain, the resultant diploid was found to be HMLα/HMLα MATa/MATαHMRa/HMRα, indicating that conversion of MATa to MATα had taken place in the course of mating. From all these observations, we conclude that there is a group of S. cerevisiae strains that carries factor(s) that induces conversion of a mating‐type cassette of the mating partner to α mating‐type cassette and that this mating type cassette conversion takes place in all three mating type loci, HML, MAT and HMR, if the loci are in the non‐silenced condition. Copyright © 2004 John Wiley & Sons, Ltd.