Forward genetics is used to identify genes that are involved in particular biological processes. For example, genes required for disease resistance can be found by identifying mutants with reduced or increased disease resistance, genes that control flower development can be identified by searching for mutants with altered flower morphology, and genes encoding enzymes for tryptophan biosynthesis can be identified by searching for mutants that require exogenous tryptophan for growth. - [Read Forward Genetics in Arabidopsis: Finding Mutations that Cause Particular Phenotypes Protocol]
Searches are not constrained for only tryptic peptides, and indexed databases (databases only containing tryptic peptides) are not used. In cases where there are very complex mixtures, such as cell lysates, nonspecific cleavages can occur. Therefore, nontryptic peptides would be missed in the database search. - [Read The Use of Mass Spectrometry in Proteomics: Database Searching]