TYMS and colonic neoplasm: When the TS6 siRNA was modified with 5 dT on the sense strand and 5 deoxyadenosine (dA) on the antisense strand to form TS6 ssiRNA, PEI was able to efficiently deliver and release TS6 ssiRNA into human colon cancer RKO cells, resulting in enhanced knockdown of TS protein levels (Figure 1, lane 5).