The epididymal secretory protein E1, also known as NPC2 (Niemann-Pick intracellular cholesterol transporter 2), is one of two main lysosomal transport proteins that assist in the regulation of cellular cholesterol by exportation of LDL-derived cholesterol from lysosomes. Lysosomes have digestive enzymes that allow it to break down LDL particles to LDL-derived cholesterol once the LDL particle is engulfed into the cell via receptor mediated endocytosis. NPC2 works cooperatively with the NPC1 protein to facilitate the exportation of LDL-derived cholesterol out of the lysosome to regulate the concentrations of lipids and cholesterol in the body. Epididymal secretory protein E1 is a protein associated with Niemann-Pick disease, type C, which is one of the 3 types of the Niemann-Pick diseases (Type A, B, and C). This disease can lead to an over accumulation of cholesterol and lipids in different types of tissues, including the brain. It is caused by a mutation in the NPC2 gene that impairs the body's ability to transport lipids or cholesterol intracellularly.