SRY‐box transcription factor 10 is a highly specific biomarker of basal‐like breast cancer.

Aims: Basal‐like breast cancer is an aggressive molecular subtype associated with younger age and early relapse. Most cases lack expression of oestrogen receptor (ER), progesterone receptor, and human epidermal growth factor receptor 2, limiting targeted therapeutic options. Basal‐like breast cancer is defined by the expression of genes in the outer/basally located epithelial layer of mammary glands, including those encoding cytokeratin (CK) 5 and CK14, and epidermal growth factor receptor (EGFR). SRY‐box transcription factor 10 (SOX10), for which there is a readily available immunohistochemical stain, is expressed in a subset of breast cancers, particularly triple‐negative carcinomas. In this study, we sought to: (i) assess the association between SOX10 expression and intrinsic molecular subtypes as defined by Prediction Analysis of Microarray 50 (PAM50) gene expression; and (ii) compare the performance of SOX10 with that of other surrogate markers of the basal‐like subtype, including CK5, EGFR, nestin, and inositol polyphosphate 4‐phosphatase type II (INPP4B). Methods and results: SOX10 immunostaining was performed on tissue microarrays constructed from a contemporary series enriched for ER‐negative and weakly ER‐positive cancers that had also undergone PAM50 gene profiling. A total of 211 cases were informative for both SOX10 immunohistochemistry and PAM50 subtype, including 103 basal‐like cancers. Staining for SOX10 was positive in 73 of 103 basal‐like cancers and in only two of 108 cancers of other subtypes (P < 0.001), resulting in a sensitivity of 70.9% and a specificity of 98.1%. SOX10 was more specific than the other tested basal markers, and the results were independent of ER status. Conclusions: SOX10 is a moderately sensitive, but highly specific, immunohistochemical biomarker for the basal‐like intrinsic subtype of breast cancer, which, unlike other commonly used immunohistochemical biomarkers, is independent of hormone receptor status. [ABSTRACT FROM AUTHOR]