Simultaneous costaining of breast cancer ZR75 cells (Fig 5, panels E) with anti-MUC1-TM mAbs [DMB5F3 [17], directly labeled with a green fluorescent dye] together with anti-MUC1-ARF mAb 2G10 (labeled red), clearly showed that the two MUC1 proteins did not colocalize: MUC1-TM localized at the cell membrane as expected, whereas MUC1-ARF protein was predominantly nuclear (Fig 5, see panels Ei, Eii and Eiii). This evidence concerns the gene MUC1 and breast carcinoma.