We sent out a protein that was N-terminally blocked, and it was digested with modified sequencing grade trypsin (Boehringer Mannheim) in the gel; peptides were separated by HPLC. The reference to this procedure which we were given: Rosenfeld, J., Capdeviell, J., Guillemot, J., and Ferrara, P. (1992) Anal. Biochem. 203, 173-179