Normal prelactating mammary gland in a mid-pregnant FVB female stained using the immunoperoxidase technique to identify CD31 (PECAM). Note that the vessels have a delicate red-brown cytoplasmic stain. The antibody identifies some but not all endothelial cells in the mammary gland. Compare this pattern with the IHC of SMA and laminin. The primary antibody is a polyclonal goat antibody against human PECAM.