We used positional cloning to identify the circadian Clock gene in mice. Clock is a large transcription unit with 24 exons spanning ∼100,000 bp of DNA from which transcript classes of 7.5 and ∼10 kb arise. Clock encodes a novel member of the bHLH–PAS family of transcription factors. In the Clock mutant allele, an A→T nucleotide transversion in a splice donor site causes exon skipping and deletion of 51 amino acids in the CLOCK protein. Clock is a unique gene with known circadian function and with features predicting DNA binding, protein dimerization, and activation domains. CLOCK represents the second example of a PAS domain–containing clock protein (besides Drosophila PERIOD), which suggests that this motif may define an evolutionarily conserved feature of the circadian clock mechanism.