TIMP1 and breast cancer: Given that too high, or too low, endogenous cellular levels of TIMP‐1 could confound our analyses, and that presence of CD63 would obscure the interpretation of experiments aiming to ascertain a role for CD74 in internalization of TIMP‐1, we chose as model system a breast cancer cell line with moderate‐to‐high expression of CD74, with concomitant moderate expression of TIMP‐1, and undetectable expression of CD63.