S. pombe is a species of yeast, also called fission yeast, which is used as a model organism for biological and medical study. They are useful because most of the time they are haploid so that both recessive and dominant mutations in the genome will result in a mutant phenotype (since each individual yeast cell has only one copy of each gene, unlike in diploid organisms where a normal copy of a gene can make up for recessive mutations, resulting in mutant phenotypes only for dominant mutations).