Skin Melanomas 
Several systems are used to stage skin melanomas. The depth of invasion is the best predictor of outcome. One classification categorizes primary melanomas into five levels of invasion that a pathologist can easily recognize under the microscope (Clark's levels).
• Level 1 is melanoma at the place of
  origin, in the basal layer of the epidermis
  at the epidermal-dermal junction, where
  the outer and underlying layers of the
  skin meet.
• Level 2 is extension to the upper third
  of the dermis, the papillary dermis.
• Level 3 melanoma extends to the border
  of the papillary and reticular dermis.
• Level 4 involves the reticular dermis. 
• Level 5 invades subcutaneous tissue ,
  such as fat.