To evaluate a potential functional interrelation of Mcl-1 and Her2 in Her2-positive BC cells under hypoxic conditions, we next transfected the Her2-positive BC cell lines BT-474 and SKBR3 with a pool of siRNA directed against the MCL1 gene (siMCL1) and tested for both Her2 and Hif-1α protein levels. This evidence concerns the gene HIF1A and breast cancer.