The invention relates to the recombinant expression of a peptide of interest in the form of a fusion protein comprising a solubility tag. The fusion protein comprises at least two portions separated by a cleavable peptide sequence wherein one portion is devoid of cysteine residues and the second portion comprises an effective number of cross-linkable cysteine residues. After cell lysis and isolation of the fusion protein, the fusion protein is subsequently cleaved into a mixture of first and second portions. Oxidative cross-linking is used to selectively precipitate one of the two portions to facilitate simple and effective separation of the peptide of interest.