U1A binds directly to the SMN 3′-UTR with high affinity and specificity. Gel shift assays of radiolabeled SMN RNA incubated with recombinant U1A protein were performed. A, 10-fold titrations of U1A protein with SMN wild type 3′-UTR. B, 10-fold dilutions of U1A protein with SMN site 1 mutant. C, 10-fold dilutions of U1A with the SMN site 1 and site 2 mutants. D, 2-fold dilutions of U1A protein with SMN wild type 3′-UTR under equilibrium conditions. E, 2-fold dilutions of U1A protein with SMN site 1 mutant under equilibrium conditions. F, binding curve of experiment in D with Kd.