Tuberous sclerosis 1 (TSC1), also known as hamartin, is a protein that in humans is encoded by the TSC1 gene. TSC1 functions as a co-chaperone which inhibits the ATPase activity of the chaperone Hsp90 (heat shock protein-90) and decelerates its chaperone cycle. Tsc1 functions as a facilitator of Hsp90 in chaperoning the kinase and non-kinase clients including Tsc2, therefore preventing their ubiquitination and degradation in the proteasome. TSC1, TSC2 and TBC1D7 is a multi-protein complex also known as the TSC complex. This complex negatively regulates mTORC1 signaling by functioning as a GTPase-activating protein (GAP) for the small GTPase Rheb, an essential activator of mTORC1. The TSC complex has been implicated as a tumor suppressor. Defects in this gene can cause tuberous sclerosis, due to a functional impairment of the TSC complex. Defects in TSC1 may also be a cause of focal cortical dysplasia. TSC1 may be involved in protecting brain neurons in the CA3 region of the hippocampus from the effects of stroke.