Sukswai N, Aung PP, Cameron YC, et al. - Researchers investigated if a reliable and practical marker for blastic plasmacytoid dendritic cell neoplasm (BPDCN) in biopsy material could be assessed via a tissue-based assay designed to detect dual CD123 and TCF4 expression. A dual-color TCF4/CD123 immunohistochemistry stain was designed, optimized, and validated for use in formalin-fixed paraffin-embedded tissue sections. In 48 confirmed BPDCN cases, confirmation of the performance characteristics of the TCF4/CD123 stain was made. Plasmacytoid dendritic cells showed reproducible TCF4/CD123 coexpression. In BPDCN, coexpression was detected in all (48/48; 100%) cases using the TCF4/CD123 stain. Comparable staining characteristics were observed in cases with concurrent samples from different anatomic sites. In contrast, of 464 non-BPDCN cases comprising a wide range of hematolymphoid neoplasms and cutaneous lesions that might enter in the differential diagnosis of BPDCN, dual expression of TCF4 and CD123 was observed in only 1 case of B-lymphoblastic leukemia/lymphoma. These findings suggest analytic sensitivity of 100% and a specificity of 99.8% of the TCF4/CD123 dual-color immunohistochemical stain. For the diagnosis of BPDCN, the dual-color TCF4/CD123 immunohistochemistry stain represents a robust standalone and cost-effective assay.