This work describes a multifunctional phage lambda expression vector system, lambda YES, designed to facilitate gene isolation from eukaryotes by complementation of Escherichia coli and Saccharomyces cerevisiae mutations. lambda YES vectors have a selection for cDNA inserts using an oligo adaptor strategy and are capable of expressing genes in both E. coli and S. cerevisiae. They also allow conversion from phage lambda to plasmid clones by using the cre-lox site-specific recombination system, referred to here as automatic subcloning. A simple method has been developed for the conversion of any plasmid into a phage lambda cDNA cloning vector with automatic subcloning capability. cDNA libraries constructed in these vectors were used to isolate genes from humans and Arabidopsis thaliana by complementation of yeast and bacterial mutations, respectively.