Directionally cloned random cDNA expression vector libraries, compositions and methods of use

A method for producing a vector, comprising:

• a) contacting an mRNA with a random primer under conditions suitable for production of a double-stranded cDNA, said random primer having at its 5′ terminus a partial sequence of a restriction site for an infrequently cutting restriction endonuclease;
• b) ligating double-stranded adaptors to the ends of said double-stranded cDNA to produce an adaptor-modified cDNA having a complete restriction site sequence for said endonuclease at one end of said double-stranded cDNA;
• c) contacting said adaptor-modified cDNA with said restriction endonuclease to produce digested cDNA;
• and
• d) ligating the digested cDNA into a vector.