Using Siglec-5 aptamer K19 to enhance the saporin toxicity for inhibiting NB4 cell proliferation in vitro. Various concentrations of biotinylated-aptamer K19 (Biotin-K19) or biotinylated single-stranded negative control DNA (Biotin-SS DNA) and streptavidin-saporin (SA-SAP) or free saporin (SAP) were mixed, and were then incubated with NB4 cells (a and b). The non-biotinylated aptamer K19 (K19) was used to block the binding of biotin-K19 to NB4 cells (c). The viable cells were then measured in 72 hours. The experiments were performed in triplicates, and the data is shown as fractions of the control groups. The P values are given as “*” or “**” representing the P values of < 0.05 or < 0.01, respectively.