Progesterone receptor membrane component 1 (PGRMC1) is a heme binding protein implicated in a wide range of cellular functions. Our previous studies showed that PGRMC1 binds to cytochromes P450 in yeast and mammalian cells and promotes activity of these enzymes. Recently, the paralog PGRMC2 was shown to function as an intracellular heme chaperone. Here, we examined the function of Pgrmc1 in mouse liver by measuring levels of membrane proteins in livers from wild-type and Pgrmc1 knockout mice. These experiments revealed that Pgrmc1 knockout mouse livers contained reduced levels of cytochrome P450 enzymes.