Binding of U2AF65 to human PY tracts validates the U2AF65 SELEX scoring system. (a) Gel shift of human U2AF65 with human PY tract RNA oligonucleotides. (b) RNA sequences used for binding studies. The gene and intron (IVS) of origin are indicated. The Kd values are the average of triplicate experiments. Kd values marked with an asterisk are estimated since the levels of protein required to reach saturation exceed the capacity of the experiment. (c) Linear regression of the observed U2AF65 affinities versus the predicted S65 score.