We have earlier learnt that from whole genome sequences, functional genes are identified as open reading frames (ORFs) having initiation, and termination codons at its two ends. It has, however, been shown that an ORF does not always represent a functional gene. Therefore, verification of a gene product by the methods of proteome analysis serves a very useful purpose for 'annotation of the genome'.

Moreover, any post-translational modification of the proteins can be examined only by proteomic methods. Similarly, in expression studies, m-RNA levels may not correlate with protein levels thus making it necessary to examine protein expression levels. Protein function is also regulated by proteolysis and recycling or sequestration of proteins in various cell" compartments, and finally protein-protein interactions also determine the function of gene and the corresponding gene product, the protein.