RNF43 (ring finger protein 43), also known as URCC or RNF124, is a 783 amino acid single-pass type I membrane protein of the Endoplasmic reticulum and nuclear envelope that is thought to function as an E3 ubiquitin-protein ligase. Containing one RING-type zinc finger and existing as four alternatively spliced isoforms, RNF43 is expressed in fetal kidney and lung, as well as in cancers of the lung, colon and kidney. RNF43 functions as a cytotoxic T lymphocyte tumor antigen, and is therefore considered a target for cancer immunotherapy.