mice with the homozygous mutation of "ovtm", free run with an intrinsic period of 26 hours. "overtime" is a loss of function mutation caused by a substitution of isoleucine to threonine in the region of fbxl3 that binds to cry. in mice with this mutation, levels of the proteins per1 and per2 are decreased, while levels of cry proteins do not differ from those of wild type mice. the stabilization of cry protein levels leads to continued repression of "per1" and "per2" transcription and translation.