With the exception of CA IX, they are all predominantly expressed in differentiated tissues and some isoforms are also present in certain tumors, including CA II in GIST and endothelium of brain tumors and CA XII in kidney, breast cancer etc. (Kivelä et al., 2000; Parkkila et al., 2000, 2010; Haapasalo et al., 2008; Korhonen et al., 2009). This evidence concerns the gene CA9 and breast cancer.