TUSC5 (tumor suppressor candidate 5), also known as protein located at seventeen-p-thirteen point three 1, LOST1 or IFITMD3 (interferon-induced transmembrane domain-containing protein D3), is a 177 amino acid multi-pass membrane protein that belongs to the CD225 family. Thought to play a role in fat metabolism, TUSC5 is highly expressed in mammary gland, heart, smooth muscle, skeletal muscle and stomach, with lower levels found in lung and brain.