CXCR4 and neuroendocrine carcinoma: Additional useful markers in this diagnostic setting include p53 (missense-mutation and null-pattern staining support a diagnosis of NEC), Rb (loss of staining supports a diagnosis of NEC), SSTR2A (diffuse, strong staining favors a diagnosis of NET, though up to one-third of NECs demonstrate strong staining), and CXCR4 (diffuse, strong staining favors a diagnosis of NEC, though this marker is not widely available).