Heterozygotes have one fused digit on the hind feet, absent terminal phalanx of the first or first and second digit of the hind feet, and normal fore feet. Homozygotes lack the most anterior digit of both fore feet and hind feet, and are sterile
The Hdlk mutation arose spontaneously in a transgenic stock that had 129S1/Sv, 129X1/SvJ, and ICR in its genetic background and had been sibling inbred for over 10 generations at The Jackson Laboratory when the Hdlk mutation arose. The parental transgenic stock was maintained segregating for the X-linked transgenic insertion and this was selected out of the mutant subline (stock# 010982) through selective sibling inbreeding. In 2010 sperm were cryopreserved from heterozygous males at generation F17, and the assigned oocyte donor for cryorecovery of this bankstock is C57BL/6J.