The warning signs of skin cancer. Like many cancers, skin cancers – including melanoma, basal cell carcinoma and squamous cell carcinoma – start as precancerous lesions. These precancerous lesions are changes in skin that are not cancer but could become cancer over time.ike many cancers, skin cancers – including melanoma, basal cell carcinoma and squamous cell carcinoma – start as precancerous lesions.