Direct evidence for the requirement of delay in feedback repression in the 
mammalian circadian clock has been elusive. Cryptochrome 1 (Cry1), an essential 
clock component, displays evening-time expression and serves as a strong 
repressor at morning-time elements (E box/E' box). In this study, we reveal that 
a combination of day-time elements (D box) within the Cry1-proximal promoter and 
night-time elements (RREs) within its intronic enhancer gives rise to 
evening-time expression. A synthetic composite promoter produced evening-time 
expression, which was further recapitulated by a simple phase-vector model. Of 
note, coordination of day-time with night-time elements can modulate the extent 
of phase delay. A genetic complementation assay in Cry1(-/-):Cry2(-/-) cells 
revealed that substantial delay of Cry1 expression is required to restore 
circadian rhythmicity, and its prolonged delay slows circadian oscillation. 
Taken together, our data suggest that phase delay in Cry1 transcription is 
required for mammalian clock function.