Sphingomyelin phosphodiesterase, also known as acid sphingomyelinase and encoded by the SMPD1 gene, is a lysosomal phosphodiesterase which belongs to the acid sphingomyelinase family. SMPD1 catalyzes the hydrolysis of sphingomyelin to ceramide and phosphorylcholine. Ceramide, a bioactive lipid, has emerged as an important signaling molecule involved in a variety of cellular processes such as cell differentiation, apoptosis, and proliferation. Activation of SMPD1 occurs by the removal, chemical modification or dimerization of its C-terminal cysteine residue. Deficiencies of SMPD1 result in a lysosomal storage disorder referred to as Niemann-Pick disease. rhSMPD1 was expressed without the last three C-terminal residues, and is therefore constitutively active.