PIWIL1 and infertility disorder: For example, null mutations in each of the three piwi-coding genes lead to sterility in male mice16–18; depletion of the single functional PIWI protein in C. elegans leads to reduced fecundity19; in humans, a mutation blocking the ubiquitination of the PIWI protein HIWI has been implicated in azoospermia20.