CRLF1 is a 422 amino acid secreted protein that interacts with cells expressing ciliary neurotrophic factor receptors. A cytokine receptor subunit belonging to the type I cytokine receptor family and type 3 subfamily, CRLF1 is thought to play a role in fetal nervous system development and immunity. CRLF1 is highly expressed in stomach, placenta, heart, ovary, thyroid, bone marrow, appendix, lymph node, spleen, thymus and fetal lung, and promotes neuronal cell survival. CRLF3 is a 442 amino acid protein that contains one fibronectin type-III domain. CRLF3 is expressed in lesion actinic keratosis (AK) and skin and squamous cell carcinoma (SCC), and is thought to negatively regulate the G0/G1 phase of the cell cycle.