AbstractA cross between a sir4‐11 strain (sir4‐11 HMLα MATα HMRa, non‐mating type) and an a‐mating strain (SIR+ HMLα MATa HMRa) of Saccharomyces cerevisiae forms diploid clones at a frequency of 5 × 10−6, but the obtained diploid clones often (>70%) have altered forms of the HMRa‐containing restriction fragment, designated @ HMRa′. We previously found that some HMRa′s are associated with the conversion of HMRa to HMRα. In this report, we present evidence that another @ HMRa′ associates with the insertion of Ty into Ya of HMR. We also found that the sir4‐11 strain increased mating frequency by UV irradiation to a level of 9 × 10−4, and that generation of HMRa′ was completely prevented by disruption of RAD52 of the sir4‐11 strain. Hence, we conclude that the mutations that cause generation of HMRa′ occur in the sir4‐11 strain prior to mating. Due to these mutations, the sir4‐11 strain converts to α‐mating type and readily mates with the a‐mating strain. We discuss the usefulness of the sir4‐11 strain for the study of mutations in the HMR locus of S. cerevisiae. Copyright © 2007 John Wiley & Sons, Ltd.