Abstract: To provide a differentiation marker gene set which, in the differentiation or classification of a breast cancer subtype, can perform differentiation or classification with high reproducibility by gene expression analysis. The above-described problem is solved by a differentiation marker gene set for differentiating or classifying a subtype of breast cancer. The differentiation marker gene set comprises a combination of genes obtained by selecting at least one gene from each gene group of at least one gene group selected from gene groups composed of groups a to o, the at least one gene group being selected in accordance with a desired subtype to be differentiated or classified.

Abstract: An object of the present invention is to provide a novel internal standard gene for gene expression analysis and to provide a gene expression analysis method using the internal standard gene. The present invention provides a gene expression analysis method for a test sample, comprising the steps of: (a) measuring an expression level of a desired gene; (b) measuring at least one internal standard gene selected from the group consisting of ABCF3, FBXW5, MLLT1, FAM234A, PITPNM1, WDR1, NDUFS7, and AP2A1; and (c) comparing the expression level of the desired gene with the expression level of the internal standard gene.