Examples of positive and negative RB expression by immunohistochemistry. (A and B, 100×) Positive RB expression. The nuclei of non-neoplastic colonic mucosa and submucosa (A) and colonic adenocarcinoma (B, white arrow) exhibit diffuse strong nuclear labeling. (C, 100×) Loss of RB expression in lung adenocarcinoma. Loss of nuclear staining in neoplastic cells (blue counterstain nuclei, black arrow), while adjacent non-neoplastic stromal cells and lymphocytes have retained nuclear expression (dark brown nuclei, white arrow).