FAQ: Can cDNA libraries be cloned into M13KE?

In general, M13 is not amenable to cDNA expression, due to the requirement for in-frame expression between the leader sequence (required for secretion) and the N-terminus of coat protein pIII or pVIII. The consequence of this requirement is that an insert must be in the correct reading frame at both ends (p = 1/9) and contain no in-frame stop codons (p = [61/64] n/3, where n is the average insert length in base pairs) in order for the corresponding protein sequence to be properly fused to the coat protein. This results in a vanishingly small number of productive clones in M13 cDNA libraries. In contrast, expression of cDNA inserts as C-terminal coat protein fusions is possible in the phage T7 display systems.