S3I-201 inhibited Stat3 homodimerization, DNA binding, induction of cyclin D1, Bcl-xL and survivin and induced apoptosis of v-Src-transformed NIH3T3 cells and breast cancer cell lines with constitutively active Stat3 in the 30 to 100 μM range. Here, CCND1 is linked to breast carcinoma.