Tuberin, or TSC2 (Tuberous sclerosis complex), is implicated as a tumor suppressor. It may function in vesicular transport, and may also play a role in the regulation of cell growth arrest and in the regulation of transcription mediated by steroid receptors. Interaction between hamartin (TSC1) and tuberin may facilitate vesicular docking. It specifically stimulates the intrinsic GTPase activity of the Ras related protein RAP1A and RAB5, suggesting a possible mechanism for its role in regulating cellular growth. Mutations in tuberin lead to constitutive activation of RAP1A in tumors. At least three isoforms of Tuberin exist.