Membrane-bound transcription factor peptidase site 1 also known as Site-1 Protease (S1P) or Subtilisin kexin isozyme-1 (SKI-1) is an enzyme (EC 3.4.21.112) encoded by the MBTPS1 gene which cleaves the endoplasmic reticulum loop of sterol regulatory element binding protein (SREBP) transcription factors.