PDZK4, also known as PDZRN4L (PDZ domain-containing RING finger protein 4-like protein) or LU1, is a 769 amino acid coiled-coil protein that contains one PDZ (DHR) domain. Encoded by a gene that maps to human chromosome Xq28, PDZK4 is conserved in dog, cow, mouse, rat and zebrafish. PDZK4 localizes to cytoplasm and is expressed specifically in adult and fetal brain. PDZK4 functions as an oncogene and is up-regulated in synovial carcinomas. Treatment of synovial sarcoma cells with small interfering RNA (siRNA) inhibits PDZK4 expression, resulting in tumor-cell growth suppression, suggesting that inappropriate expression of PDZK4 may play a role synovial sarcoma cell proliferation.The exact function of FRMPD2 is unknown. The protein contains a FERM domain: such structures are often involved in signal transduction pathways. Alternatively spliced transcript variants encoding different isoforms have been identified.