Background. The alpha-test allows to detect inherited genetic changes of different types, as well as phenotypic expression of primary DNA lesions before the lesions are fixed by repair. Here we investigate ability of the alpha-test to detect base modifications induced by 6-N-hydroxylaminopurine (HAP) and determine frequency of inherited and non-inherited genetic changes in yeast strains treated with HAP. Materials and methods. The alpha-test is based on mating type regulation and detects cell type switch from α to a in heterothallic yeast Saccharomyces cerevisiae. The frequency of mating type switching reflects level of both spontaneous and induced by a mutagen DNA instability. The alpha-test may be performed in two variants: “illegitimate” hybridization and cytoduction. Conducting both complementary tests and analysis of phenotypes of the “illegitimate” hybrids and cytoductants allows to detect the full spectrum of genetic events that lead to mating type switching, such as chromosome III loss and chromosome