The cellular tumour antigen p53 is a protein found in elevated levels in a great variety of transformed cells (reviewed in ref. 1). Overproduction of p53 was observed in cells transformed by a wide spectrum of agents as well as in embryonal carcinoma cells, and in spontaneous transformants. Although initially described in mice, similar p53-like proteins were also observed in cells of other species, including those derived from several human tumours. In non-transformed cells the protein turns over very rapidly and its levels appear to correlate with cell proliferation. Thus far, very little has been known about the precise nature of the protein and of the corresponding genes. We now provide evidence for the existence of a single functional gene for murine p53 and a processed pseudogene. The predicted amino acid sequence of murine p53 is also presented.