The piwi (sometimes also PIWI; originally P-element induced wimpy testis in Drosophila) class of genes was originally identified as encoding regulatory proteins responsible for maintaining incomplete differentiation in stem cells and maintaining the stability of cell division rates in germ line cells. Piwi proteins are highly conserved across evolutionary lineages and are present in both plants and animals. One of the major human homologues, whose upregulation is implicated in the formation of tumours such as seminomas, is called hiwi; other variants on the theme include the miwi protein in mice.