Pimonidazole is an exogenous nitro-imidazole marker, which can be detected through immunohistochemical analysis of frozen sections. It detects cellular hypoxia upon becoming reduced in cells with low oxygen tension, a property that can be detected through antibody mediated detection of the reduced form. It has also shown to reliably and specifically stain hypoxic regions within the tumor, and to correlate well with patient prognosis and treatment outcome.