When SHH reaches its target cell, it binds to the Patched-1 (PTCH1) receptor (Process "3" on Figure 5, the blue molecule). In the absence of ligand, PTCH1 inhibits Smoothened (SMO), a downstream protein in the pathway (Process "4"). It has been suggested that SMO is regulated by a small molecule, the cellular localization of which is controlled by PTCH. PTCH1 has homology to Niemann-Pick disease, type C1 (NPC1) that is known to transport lipophilic molecules across a membrane. PTCH1 has a sterol sensing domain (SSD), which has been shown to be essential for suppression of SMO activity. A current theory suggests that PTCH regulates SMO by removing oxysterols from SMO. PTCH acts like a sterol pump and removes oxysterols that have been created by 7-dehydrocholesterol reductase. Upon binding of a Hh protein or a mutation in the SSD of PTCH, the pump is turned off allowing oxysterols to accumulate around SMO.