MTP1 is also highly expressed on macrophages where it mediates iron efflux from the breakdown of haem.<br>The human gene SLC40A1 encodes a metal transporter protein MTP1 (also called ferroportin or IREG1) (Schimanski LM et al, 2005). MTP1 colocalizes with ceruloplasmin (CP) which stablizes MTP1 and is necessary for the efflux reaction to occur (Texel SJ et al, 2008). Ceruloplasmin also catalyzes the conversion of iron from ferrous (Fe2+) to ferric form (Fe3+), therefore assisting in its transport in the plasma in association with transferrin, which can only carry iron in the ferric state. As well as on macrophages, MTP1 is also highly expressed in the duodenum, placenta (where it may mediate the transport of iron from maternal to foetal circulation), in muscle and the spleen.