Loss of heterozygosity (LOH) on the long arm of chromosome 13 is common in human breast tumors, pointing to the existence of several suppressor genes in this region. LOH at 13q14 has been implicated in alterations of retinoblastoma gene (RB1) expression. However, attempts to identify a link between the absence of retinoblastoma protein expression and LOH at the RB1 locus by means of immunohistochemical techniques have produced conflicting results. Therefore, we quantified RB1 mRNA by means of reverse transcriptase-polymerase chain reaction in a large series of human sporadic primary breast tumors. RB1 gene underexpression was observed in 28 (21.7%) of 129 breast-tumor RNAs. Allelic loss at this locus correlated with RB1 mRNA underexpression (P < 10(-7)), demonstrating a causal link. These data, based on a technique other than immunohistochemistry, confirm that RB1 is the main target of the 13q14 LOH observed in human breast cancer. We also found that RB1 underexpression correlated with Scarff-Bloom-Richardson (SBR) histopathologic grade III (P = 0.033), negative estrogen-receptor status (P = 0.026) and large tumor size (P = 0.010). The latter correlation was due mainly to a high mitotic index (one of the three components comprising SBR grade), suggesting that RB1 influences the proliferation rate of breast tumors. RB1 status (underexpression vs. normal expression) was not associated with subsequent relapse or with shorter relapse-free survival. This study shows a major role of the RB1 gene in the pathogenesis of breast cancer. RB1 gene underexpression promotes breast-tumor aggressiveness and rapid tumor-cell proliferation, making RB1 an outstanding target for future gene-based breast-cancer therapy.