Class E basic helix-loop-helix protein 40 (BHLHE40)

Transcriptional repressor involved in the regulation of the circadian rhythm by negatively regulating the action of the clock genes and clock-controlled genes. Acts as the negative limb of a novel autoregulatory feedback loop (DEC loop) which differs from the one formed by the PER and CRY transcriptional repressors (PER/CRY loop).

Both these loops are interlocked because it represses the expression of PER1/2 and in turn is repressed by PER1/2 and CRY1/2. Represses the action of the circadian transcriptional activator: CLOCK-ARNTL/BMAL1.