Nuclear factor (NF)-YB, a subunit of the transcription factor nuclear factor Y (NF-Y) complex, binds and activates CCAAT-containing promoters. Our previous work suggested that NF-YB may be a mediator of topoisomerase IIα (Top2α), working through the Top2α promoter. DNA topoisomerase II (Top2) is an essential nuclear enzyme and the primary target for several clinically important anticancer drugs. Our teniposide-resistant human lymphoblastic leukemia CEM cells (CEM/VM-1-5) express reduced Top2α protein compared with parental CEM cells. To study the regulation of Top2α during the development of drug resistance, we found that NF-YB protein expression is increased in CEM/VM-1-5 cells compared with parental CEM cells. This further suggests that increased NF-YB may be a negative regulator of Top2α in CEM/VM-1-5 cells. We asked what causes the up-regulation of NF-YB in CEM/VM-1-5 cells. We found by microRNA profiling that hsa-miR-485-3p is lower in CEM/VM-1-5 cells compared with CEM cells. MicroRNA target prediction programs revealed that the 3'-untranslated region (3'-UTR) of NF-YB harbors a putative hsa-miR-485-3p binding site. We thus hypothesized that hsa-miR-485-3p mediates drug responsiveness by decreasing NF-YB expression, which in turn negatively regulates Top2α expression. To test this, we overexpressed miR-485-3p in CEM/VM-1-5 cells and found that this led to reduced expression of NF-YB, a corresponding up-regulation of Top2α, and increased sensitivity to the Top2 inhibitors. Results in CEM cells were replicated in drug-sensitive and -resistant human rhabdomyosarcoma Rh30 cells, suggesting that our findings represent a general phenomenon. Ours is the first study to show that miR-485-3p mediates Top2α down-regulation in part by altered regulation of NF-YB.