For most drugs, the cytotoxic effect was not related to the KLK6 expression level; however, the cytotoxicity of the anti-leukemia drug AF was negatively related with KLK6 mRNA and protein levels (Figure 1F); i.e., KLK6 expression was higher in AF-resistant NCI-N87 and SNU-620 than in AF-sensitive AGS and SNU-216 cells (Figure 1D and 1F). Here, KLK6 is linked to leukemia.