Sometimes, the term "precancer" is also used for carcinoma in situ, which is a noninvasive cancer that has not grown and spread to nearby tissue, unlike the invasive stage.

Precancerous conditions of the skin or oral cavity can appear as visible lesions without associated pain or discomfort,[7] while precancerous conditions of the hematological system are typically asymptomatic, and in the case of monoclonal gammopathy of unknown significance, it may only rarely cause numbness and tingling in the hands and feet or difficulty with balance[8] (see: peripheral neuropathy).

[9] Similarly, sun or especially UV exposure is an important risk factor for both actinic keratosis which can progress into melanomas[10] as well as skin cancer.

However, in many cases, precancerous conditions or lesions can be sporadic and idiopathic in nature, meaning that they are not associated with a hereditary genetic risk factor to the particular cancer, nor with a direct causative agent or other identifiable cause.

[12] It is thought that cancer is always preceded by a clinically silent premalignant phase during which many oncogenic genetic and epigenetic alterations accumulate before it is truly malignant.