FOLR1 and cancer: Expression of folate receptor alpha (FR‐α) in UMUC3 cells was verified using immunostaining (Figure S7 Supporting Information section); expression of FR‐α on MDA‐MB‐231 cells has been previously established.44, 45 Competitive inhibition studies are often employed to elucidate the role of a receptor in facilitating selective uptake of molecular cargo.39, 40, 47 We therefore pre‐treated cancer cells with free FA in media for 6 hr prior to delivery of polyplexes.