text: CK1δ and CK1ε were thought to be generally redundant in circadian cycle length and protein stability. Recent research, however, has shown that CK1δ deficiency lengthens circadian period while CK1ε deficiency does not. Also, CK1α has recently been suggested to play a role redundant to CK1δ in phosphorylating PER1 although this is not consistent with other data