I have cloned (correctly) a DNA sequence in the expression vector pET22b using Nco - XhoI . When I express the 26KdA recombinant protein periplasmatically I seem to obtain a fairly good yield of full length protein, but on controlling the mass of the protein expressed I find that it corresponds exactly to a protein 9 aminoacids shorter (at the N-terminal end). Is it possible that there is miscleavage of the leader sequence and it actually cleaves a sequence 9 aminoacids longer than expected ?