A common gene-expression signature for IgM⁺IgD⁺CD27⁺ B cells from blood and spleen. Each column represents microarray data from a sample of the indicated cell subtype, and each row represents the expression of a single gene. The spleen IgD⁺CD27⁺ and IgD^-CD27⁺ populations are obtained from 2 separate donors, with 1 of the 2 samples prepared in duplicate. Red squares indicate increased expression and green squares indicate decreased expression relative to the median expression of the gene according to the color bar shown. Gray squares indicate missing or excluded data. (A) The array dendrogram obtained by clustering the 49 genes that differentiated (P < .005, 2-fold higher expression) the splenic IgD⁺CD27⁺ samples from the splenic IgD^-CD27⁺ samples. The red branches indicate the coclustering of the blood IgD⁺CD27⁺ samples, with the splenic IgD⁺CD27⁺ samples, and the blue branches indicate the coclustering of the blood IgD^-CD27⁺ samples with their respective splenic IgD^-CD27⁺ samples. (B) The 37 genes that achieved statistical significance with a 2-fold higher mean expression when the IgD⁺CD27⁺ cell populations are compared with the memory IgD^-CD27⁺ cell populations.