Effect of conservation of essential genes on probability of essentiality. This graph shows the likelihood of essentiality for C. elegans genes with essential orthologues in one or more of the organisms; D. melanogaster, M. musculus and S. cerevisiae. The percentages are the number of genes with lethal phenotypes divided by the total number of genes with orthologues in that species. Essentiality was assessed for genes without paralogues. C = C. elegans, D = D. melanogaster, M = M. musculus, S = S. cerevisiae.