We have made a large scale analysis of prototrophic products of spontaneous and induced mitotic recombination within LYS2 gene of the yeast Saccharomyces. The mutant alleles staying in heterozygote with the wild type allele were uncovered and analysed. Among thirteen lys2 mutations used in the study three had reduced frequencies of mitotic gene conversion. These rarely converting mutations gave a remarkably high proportion of reciprocal events (up to 38%) in pairwise combinations, never seen for any other pair of alleles studied. Two of these mutations are the deletions of large parts of LYS2 gene. The results suggest that mispairing in the region of deletion blocks the hybrid DNA migration and leads to the reduced conversion ability of deletions. Comparison of uncovered alleles ratio in all allele combinations tested lead us to another hypothesis about bidirectional migration of hybrid DNA.