WPO term usage. Shown is the distribution of the number of phenotypes (y-axis) with the indicated number of genes annotated per phenotype term (x-axis). Of the 1880 phenotype terms in the WPO, 486 (26%) are unused. Of the remaining terms, 684 have been used to annotate between 1 to 5 genes. 253 terms have been used to annotate between 6 and 10 genes, and so on. The most used phenotype term is 'embryonic lethal', which has been used to annotate 3304 genes (not shown, 'embryonic lethal' is one of 8 terms that have been used to annotate greater than 1000 genes).