Such examples include immunohistochemistry (IHC) for cyclin D1 overexpression in mantle cell lymphoma, PD1 expression in clonally‐related large B‐cell lymphoma (Richter's) transformation from CLL, BRAF V600E IHC detection in HCL or certain histiocytic neoplasms, and IHC stains recognizing mutant NPM1 and CALR proteins. The gene discussed is BRAF; the disease is hairy cell leukemia.