SPCS3 (signal peptidase complex subunit 3), also known as SPC3, is a 180 amino acid single-pass type II membrane protein that localizes to both the microsome and the endoplasmic reticulum (ER) and belongs to the SPCS (signal peptidase complex subunit) family. Existing as a component of the microsomal signal peptidase complex which consists of five members, SPCS3 functions to remove signal peptides from proteins that are translated to the lumen of the ER. The gene encoding SPCS3 maps to human chromosome 4, which encodes nearly 6% of the human genome and has the largest gene deserts (regions of the genome with no protein encoding genes) of all of the human chromosomes. Defects in some of the genes located on chromosome 4 are associated with Huntington's disease, Ellis-van Creveld syndrome, methylmalonic acidemia and polycystic kidney disease.