While Cyclin G1 is a transcriptional target of p53 (TP53), the major tumor suppressor, the CCNG1 gene is overexpressed in many cancer cells in the absence of a functional p53, which led to the hypothesis and eventually the experimental confirmation that Cyclin G1 (CCNG1) represents a valid, proximal, and transforming proto-oncogene in its own right. The gene discussed is TP53; the disease is cancer.