POM121 (POM121 membrane glycoprotein), also known as POM121A (nuclear envelope pore membrane protein POM 121A), NUP121 (nucleoporin Nup121) or pore membrane protein of 121 kDa, is a 1,249 amino acid single-pass membrane protein that exists as three alternatively spiced isoforms and belongs to the POM121 family. Encoded by a gene that maps to human chromosome 7q11.23, POM121 is highly conserved and localizes to nucleus membrane and endoplasmic reticulum membrane. An essential component of the nuclear pore complex (NPC), POM121 is mandatory for nuclear envelope formation and may play a role in biogenesis of the NPC. POM121 associates with the central spoke ring complex and is involved in anchoring components of the pore complex to the pore membrane by way of its F-X-F-G repeat-containing domain. POM121 overexpression induces the formation of cytoplasmic annulate lamellae (AL).