The active factor is excreted into the culture medium by haploid cells of the alpha mating type and acts on cells of the opposite mating type (type A). It mediates the conjugation process between the two types by inhibiting the initiation of DNA synthesis in type a cells and synchronizing them with type alpha. The mating factor alpha-1 precursor is identical in S.italicus, S.uvarum and S.cerevisiae, except for the number of tandem repeat units: 5, 3 and 4 respectively.