We present an immunosensor for the rapid and sensitive detection of the p53 oncosuppressor protein and of its mutated form p53 R175H, which are both valuable cancer biomarkers. The sensor is based on the accurate measurement of the source-drain current variation of a metal oxide semiconductor field-effect transistor, as due to the gate potential changing arising from charge release upon the selective capture of a biomarker by the partner immobilized on a sensing surface connected to the gate electrode. A suitable microelectronic system is implemented to combine high current resolution, which is needed to be competitive with standard immunoassays, with compact dimensions of the final sensor device.