Puf proteins bind RNA sequence specifically and regulate translation and 
stability of target mRNAs. A "code" for RNA recognition has been deduced from 
crystal structures of the Puf protein, human Pumilio1, where each of eight 
repeats binds an RNA base via a combination of three side chains at conserved 
positions. Here, we report the creation of seven soluble mutant proteins with 
predictably altered sequence specificity, including one that binds tightly to 
adenosine-uracil-rich element RNA. These data show that Pumilio1 can be used as 
a scaffold to engineer RNA-binding proteins with designed sequence specificity.