Heme-binding protein which promotes neuronal but not astrocyte differentiation. The cytochrome b5 heme-binding domain was proven to bind heme, although it lacks the conserved iron-binding His residues at position 73 and 106. Non-classical progesterone receptors involved in extranuclear signaling are classified in 2 groups: the class II progestin and adipoQ receptor (PAQR) family (also called mPRs) (PAQR5, PAQR6, PAQR7, PAQR8 and PAQR9) and the b5-like heme/steroid-binding protein family (also called MAPRs) (PGRMC1, PGRMC2, NENF and CYB5D2). Belongs to the cytochrome b5 family. MAPR subfamily.