To study the role of the alternatively-spliced STAT3 isoforms (Figure 1) in cancer cells we silenced the mRNA expression of either STAT3α (si-STAT3α), the main isoform, or STAT3β (si-STAT3β) or both (si-STAT3α+β) in MDA-MB-231 breast cancer cells using specific siRNA molecules. This evidence concerns the gene STAT3 and breast carcinoma.