Epithelial hyperplasia of usual type expresses both S100‐α and S100‐β in a heterogeneous pattern but ductal carcinoma in situ can express only S100‐α in a monotonous pattern

Immunohistochemical identification of myoepithelial cells using α‐smooth muscle actin provides little information about the nature of solid or quasi‐solid portions of epithelial hyperplasia and ductal carcinoma in situ (DCIS) because actin‐rich myoepithelial cells are usually demonstrated only in the stromal–epithelial junction of both lesions. We studied the differential distribution of α‐subunit (S100‐α) and β‐subunit (S100‐β) of S100 protein in actin‐negative areas of usual epithelial hyperplasia and DCIS by employing the streptavidin method with monospecific rabbit antibodies against each subunit. All usual epithelial hyperplasias ( n =17) were composed of heterogeneous epithelial cell types; cells expressing S100‐α and/or S100‐β were intermingled with non‐expressing cells, resulting in a mosaic‐like pattern. On the contrary, DCIS ( n =32) uniformly lacked immunoreactive S100‐β; S100‐α was diffusely expressed in 24 (68.8%) DCIS (three solid/comedo, 13 cribriform, four endocrine, one micropapillary, three papillary variants) and negative in the remaining eight (31.2%) DCIS (one cribriform, two micropapillary, four papillary and one apocrine variants). In conclusion, in contrast to usual epithelial hyperplasia that expresses both S100‐α and S100‐β in a heterogeneous pattern, DCIS can express only S100‐α in a monotonous pattern, possibly signifying unidirectional differentiation toward secretory glandular epithelium.