BCL9 and ductal breast carcinoma in situ: To examine a protein interaction between BCL9 and STAT3, whole-cell extracts of DCIS.COM and SUM225 were co-immunoprecipitated (Co-IP) with anti-BCL9 and anti-STAT3 antibodies followed by Western blot using anti-STAT3, anti-BCL9 and anti-P(Y705) STAT3 antibodies.