USP4 and breast cancer: To confirm complex formation of endogenous components, proximity ligation assay (PLA) was performed using the breast cancer cell line MDA-231-BM, known to produce high levels of hyaluronan.27 Using antibodies against HAS2 and USP4 or USP17, reactive signals in the form of red dots indicating interactions between endogenous HAS2 and USP17, and HAS2 and USP4, were observed; only a low background level of dots was seen when each antibody was used alone, or in the fixation control without antibody (Figure 2c).