[Isoform 1]: Heme b transporter that mediates heme efflux from the cytoplasm to the extracellular compartment. Heme export depends on the presence of HPX and is required to maintain intracellular free heme balance, protecting cells from heme toxicity. Heme export provides protection from heme or ferrous iron toxicities in liver, brain, sensory neurons and during erythropoiesis, a process in which heme synthesis intensifies. Possibly export coproporphyrin and protoporphyrin IX, which are both intermediate products in the heme biosynthetic pathway. Does not export bilirubin. The molecular mechanism of heme transport, whether electrogenic, electroneutral or coupled to other ions, remains to be elucidated.; [Isoform 1]: (Microbial infection) Confers susceptibility to feline leukemia virus subgroup C (FeLV-C) infection in vitro.; [Isoform 2]: Heme b transporter that promotes heme efflux from the mitochondrion to the cytoplasm. Essential for erythroid differentiation.