The timeless (tim) gene is essential for circadian function in Drosophila. Putative homologs of Drosophila tim have been identified in both mice and humans (mTim and hTIM, respectively). Mammalian TIM is not the true orthologue of Drosophila TIM, but is the likely orthologue of a fly gene, timeout (also called tim-2). mTim has been shown to be essential for embryonic development, but does not have substantiated circadian function. Some family members contain a SANT domain in this region.