Image 23.

p53 and Rb Immunohistochemistry in Poorly Differentiated Neuroendocrine Carcinoma: (A) Small cell lung cancer demonstrating (B) null pattern p53 staining and (C) loss of Rb protein staining. Biallelic inactivation of TP53 and RB1 is the molecular genetic hallmark of small cell lung cancer and is frequently seen in extrapulmonary visceral small cell carcinomas, as well. Although null pattern p53 staining is shown in this example, missense and gain-of-function mutations, resulting in diffuse, strong p53 staining, predominate in small cell lung cancer on the order of 3-4:1.