LDOC1 (Leucine zipper protein downregulated in cancer cells) is a 146 amino acid nuclear protein that contains a leucine zipper-like motif and a proline-rich region that shares marked similarity with an SH3-binding domain. The protein localizes to the nucleus and is downregulated in some cancer cell lines. It is thought to regulate the transcriptional response mediated by the nuclear factor kB (NF-kB). The gene has been proposed as a tumor suppressor gene whose protein product may have an important role in the development and/or progression of some cancers. LDOC1L (Leucine zipper protein down-regulated in cancer cells-like), also known as MART6 (Mammalian retrotransposon-derived protein 6), is a 239 amino acid protein that belongs to the LDOC1 family.