Breast cancer is made of various molecular subtypes with different prognosis. Using whole-genome oligonucleotide microarrays, Finetti and colleagues studied the expression of kinase genes in 80 luminal A and 58 basal breast cancers. The expression (measured by a kinase score) of 16 genes encoding serine/threonine kinases involved in mitosis distinguished two subgroups of luminal A tumors: Aa, of good prognosis, and Ab, of poor prognosis (top image). This classification and its prognostic impact were validated in 276 luminal A cases from three independent series (bottom image). Among 1222 breast cancer samples (middle image), most of the luminal A (dark blue for Aa and black for Ab) and normal-like (green) tumors had negative kinase scores, while most of the basal (red ) and luminal B (light blue) tumors had positive kinase scores. ERBB2-overexpressing (pink) samples were equally distributed with respect to their kinase scores. This mitotickinase signature reveals a continuum of luminal cases from the more proliferative (luminal B) to the less proliferative (luminal Aa). For details, see the article by Finetti and colleagues on page 767 of this issue.