Compositions and methods are provided for generating three frame cDNA expression libraries for functional screening of proteins. The invention includes sets of 5' adapters for cloning cDNA molecules in which the sets include three adapters that can be used to clone a particular cDNA in all three reading frames. The libraries so generated have greater complexity of expressed open reading frames, and thus can improve the success of functional protein screens, such as two hybrid screens. The adapters also have recombination sites for efficient cloning and transfer between systems.