Amino acid replacement: S661A.
Flies expressing perS661A.13.2.T:Ivir\HA1,T:Zzzz\His6 in a per01 mutant background exhibit free-running periods that are approximately 2 h longer compared with controls. The period lengthening effects of perS661A.13.2.T:Ivir\HA1,T:Zzzz\His6 are not observed in a wild-type genetic background.