MMP9 and cancer: In most adult tissue, SIX1 is little expressed; however, in some cancer tissues, the re-expression of SIX1 promoted cellular proliferation, migration and stem cell-like features by regulating cyclins and matrix metalloproteinase 9 (MMP9) (Coletta et al., 2004; Wang et al., 2012; Wu et al., 2015; Zhang et al., 2019).