The majority of urticaria pigmentosa cases are caused by a point mutation at amino acid 816 of the proto-oncogene c-kit.[2] c-kit is a transmembrane protein which, when bound to Mast Cell Growth Factor (MCGF), signals the cell to divide. Mutations in position 816 of c-kit can result in a constant division signal being sent to the mast cells, resulting in abnormal proliferation.