This family includes heme binding domains from a diverse range of proteins. This family also includes proteins that bind to steroids. The family includes progesterone receptors such as Swiss:O00264 [1,2]. Many members of this subfamily are membrane anchored by an N-terminal transmembrane alpha helix. This family also includes a domain in some chitin synthases. There is no known ligand for this domain in the chitin synthases.

1: Cloning and tissue expression of two putative steroid membrane receptors. Gerdes D, Wehling M, Leube B, Falkenstein E; Biol Chem 1998;379:907-911. PMID:9705155

2: Purification and partial sequencing of high-affinity progesterone-binding site(s) from porcine liver membranes. Meyer C, Schmid R, Scriba PC, Wehling M; Eur J Biochem 1996;239:726-731. PMID:8774719

3: Membrane-bound progesterone receptors contain a cytochrome b5-like ligand-binding domain. Mifsud W, Bateman A; Genome Biol 2002;3:0-0. PMID:12537557