tokens: Administration of drugs that inhibit adenine phosphoribosyltransferase ( APRT ) in HPRT - deficient mice has produced the suggestion that deficiency of APRT in combination with HPRT - deficiency in mice may lead to self - mutilation behavior [ C . L . Wu and D . W . Melton ( 1993 ) Nature Genet . 3 , 235 - 240 ] .