To test whether an increased density of detached breast cancer cells can block the activity of Mek or that of its substrate Erk and inhibit ErbB2 expression we compared the levels of Erk1 and Erk2 (the two Mek substrates [3]), phospho-Erk1 and phospho-Erk2 as well as that of ErbB2 in human breast cancer cells BT-474 detached from the ECM grown as a “sparse” culture to those in detached BT-474 cells grown at a 10 times higher concentration (which we refer to as a “dense” culture). This evidence concerns the gene MAPK3 and breast cancer.