Stage I is divided into stages IA and IB. In stage IA, the tumor is not more than 1 millimeter thick, with no ulceration (a break in the skin). In stage IB, (1) the tumor is not more than 1 millimeter thick and it has ulceration; or (2) the tumor is more than 1 but not more than 2 millimeters thick, with no ulceration.