RRM2 and fibrosarcoma: Further, to investigate the role of piR-39980/RRM2 in modulating chemoresistance in fibrosarcoma, we performed an MTT assay and found that cell viability of DOX-treated HT1080 cells was increased by 35% upon RRM2 overexpression (pcDNA3.1_RRM2 + DOX-treated cells) with respect to the control (pcDNA3.1 + DOX-treated cells) (P < 0.001, Fig. 8b).