Most proteins involved in these transcriptional feedback loops are phosphorylated in a time-of-day–dependent manner, and three kinases, casein kinase I (CKI)ɛ/δ/Doubletime (DBT), CK2, and GSK-3/Shaggy (SGG), have been identified as essential clock components in Drosophila. Importantly, a mutation of the human ortholog of DBT, CKIδ, has been associated with certain forms of a heritable disorder of sleep, familial advanced sleep phase syndrome (FASPS; [7]). Here, DBT is linked to Familial advanced sleep-phase syndrome.