MMP9 and breast cancer: USP4 can promote cancer in various ways, such as inhibiting SMAD4 monoubiquitination and promoting activin and BMP signaling (Zhou et al., 2017), promoting breast cancer metastasis via the p21-activated kinase 5-aspartyl aminopeptidase-USP4 axis (Geng et al., 2020) and promoting the invasion of breast cancer cells via the relaxin/TGF-β1/SMAD2/MMP-9 signaling pathway (Cao et al., 2016).