The "P" element encodes for the protein "P" transposase. Unlike laboratory strain females, wild type females are thought also to express an inhibitor to "P" transposase function, from the very same element. This inhibitor reduces the disruption to the genome caused by the "P" elements, allowing fertile progeny. Evidence for this comes from crosses of laboratory females (which lack "P" transposase inhibitor) with wild type males (which have "P" elements). In the absence of the inhibitor, the "P" elements can proliferate throughout the genome, disrupting many genes and killing progeny.