More than 10 genes are involved in the biosynthesis of glycosylphosphatidylinositol (GPI), which anchors many mammalian cell surface proteins to the membrane. Paroxysmal nocturnal hemoglobinuria (PNH) is caused by a somatic mutation in a GPI biosynthesis gene within the hematopoietic stem cell. The X-linked gene PIGA has been found to be mutated in all patients with PNH. This is probably because all other GPI synthesis genes are autosomal; hence two somatic mutations must occur to cause PNH, whereas one somatic mutation is sufficient to inactivate PIGA. Consistent with this notion, three other genes, PIGB, PIGF, and PIGH, are autosomal. Here we isolated a genomic clone of another GPI-synthesis gene, PIGC, and mapped it to chromosome 1q23-q25, further supporting this notion. PIGC is an intronless gene. We found an intronless pseudogene of PIGC, PIGCP1, and mapped it to chromosome 11p12-p13. The presence of a processed pseudogene is a common feature of PIGA, PIGF, and PIGC.