LDLRAP1 (Low density lipoprotein receptor adapter protein 1) is a cytosolic protein which contains a phosphotyrosine binding (PTD) domain. LDLRAP1 is a cytosolic adaptor that couples LDLR to endocytic machinery. Defects in LDLRAP1 are the cause of autosomal recessive hypercholesterolemia (ARH). ARH is a disorder caused by defective internalization of LDL receptors (LDLR) in the liver.