Timing of CAB expression 1 is a protein that in Arabidopsis thaliana is encoded by the TOC1 gene. TOC1 is also known as two-component response regulator-like APRR1. TOC1 was the first plant gene that, when mutated, yielded a circadian phenotype. It codes for the transcription factor TOC1, which affects the period of plants' circadian rhythms: built-in, malleable oscillations that repeat every 24 hours. The gene codes for a transcriptional repressor, TOC1, one of five pseudo-response regulators (PRR) that mediate the period of the circadian clock in plants. The TOC1 protein is involved in the clock's evening loop, which is a repressilator that directly inhibits transcription of morning loop genes LHY and CCA1. Toc1 gene is expressed in most plant structures and cells, and has its locus on chromosome 5.