Abstract: A mutant human insulin receptor DNA, wherein the base sequence encoding Thr.sup.831 in human insulin receptor DNA has been replaced by a base sequence encoding Ala and/or the base sequence encoding Tyr.sup.1334 therein has been replaced by a base sequence encoding Cys; a fragment of the mutant human insulin receptor DNA containing the mutation part(s); a diagnostic probe for non-insulin-dependent diabetes mellitus comprising this fragment; and a diagnostic drug for non-insulin-dependent diabetes mellitus containing this fragment.