The present invention provides methods and compositions for the diagnosis and treatment of cells lacking normal growth arresting characteristic. The present invention demonstrates that many tumor cells lack normal cell surface α-dystroglycan and thereby lack dystroglycan function. Dystroglycan can be lost from the cell surface by proteolytic shedding of a fragment of α-dystroglycan into the surrounding medium. Upon restoration of dystroglycan function and over-expression of the dystroglycan gene, the once tumorigenic cells revert to non-tumorigenic cells which polarize and arrest cell growth in the presence of basement membrane proteins, demonstrating that dystroglycan functions as a tumor marker and suppressor.