While these data do not prove that the enhanced matrix metalloproteinase activity is the mechanism of increased aggressiveness seen in Ly6a/Sca-1 high leukemia cells, they are consistent with a substantial body of research that does show MMP-9 activity is important in the determining the invasiveness of human chronic lymphocytic leukemia [29], [30]. Here, MMP9 is linked to B-cell chronic lymphocytic leukemia.