CD74 and breast cancer: Analysis of the expression levels of CD74, CD63, and TIMP‐1, respectively, in various breast cancer cell lines, including MDA‐MB‐231, MCF‐7, T47D, and SK‐BR‐3 (Fig. S1), identified MDA‐MB‐231 (high CD74; undetectable CD63; moderate TIMP‐1 expression) as the most adequate model system for us, and we used this line for all subsequent studies.