Random spore analysis (RSA) is a tool that allows for the screening of a large number of meiotic products. It requires only a limited effort, and is often the method of choice for constructing strains with unambiguous genotypes. It is also useful to identify the frequency of rare events. Strains are crossed on a nitrogen-limiting medium for three days. Mated cells are observed under the microscope to check for the presence of ripe asci. To release spores from their ascus, a sample of the cross is taken from the mating plate and resuspended in an enzyme solution overnight at 25-29 °C. Spores are then counted using a hemocytometer before plating an appropriate number. Incubation at the appropriate temperature follows until colonies form.