A process for detecting malignant transformation of cells involves detecting the overexpression of the products of the &bgr;3, &bgr;5, &bgr;8 and &bgr;9 genes, which encode the hCG&bgr; subunit, relative to their expression in nonmalignant cells. A kit for diagnosing an hCG- or an hCG fragment-secreting cancer includes an assembly of polypeptides covering at least a part of the primary sequence of hCG. The use of a polypeptide corresponding to at least one portion of the primary sequence of hCG for producing a composition useful in hCG- or hCG fragment-secreting cancer immunotherapy is also disclosed.