Recently, some studies suggested the involvement of matrix metalloproteinase 2 (MMP-2) and matrix metalloproteinase 9 (MMP-9), also known as gelatinase A and gelatinase B, respectively, in breast cancer initiation and growth throughout complex interactions with the key oncogenes and tumor-suppressor genes which are involved in the initial stage of tumorigenesis. This evidence concerns the gene MMP9 and breast cancer.