Saccharomyces cerevisiae can exist as either a haploid cell of either Mat a or Mat α mating type, a diploid cell or a polyploid cell, but the transfer of genetic material is normally restricted to conjugation between haploids of opposite mating type. It is possible, however, to isolate genetically stable yeast hybrids from strains that are unable to mate by using protoplast fusion. In this process, the yeast cell walls are enzymatically removed, the resulting protoplasts are fused together using polyethylene glycol (PEG) and calcium ions (Ca²⁺ ), before being embedded in agar under appropriate selective conditions. Colonies of genetically stable hybrids are normally visible within 5–10 d at 25�C.