Schizosaccharomyces pombe
In "Saccharomyces cerevisiae" (budding yeast) 15-20% of all genes are essential. In "Schizosaccharomyces pombe" (fission yeast) 4,836 heterozygous deletions covering 98.4% of the 4,914 protein coding open reading frames have been constructed. 1,260 of these deletions turned out to be essential.