Most TSC patients present with mutations in the TSC1 or TSC2 genes, which encode proteins that form the TSC1-TSC2 complex that serves to antagonize the signaling pathway downstream of mammalian target of rapamycin (mTOR) by promoting the activation of the small GTPase Rheb and thereby inhibiting cellular growth and proliferation. The gene discussed is TSC2; the disease is tuberous sclerosis.