Deletions and insertions of base pairs in dNA, jointly called gap events, are one of the major sources of evolutionary change at the molecular level. On the basis of very limited data, Fitch has suggested that deletions might be expressed more often than insertions in proteins. We have examined the presently available homologous protein sequences, and observed that in the course of evolution, deletions of amino acids have indeed occurred about four times more frequently than insertions. An even higher preponderance of deletions over insertions (ten times or more) is found in recent spontaneous and induced mutants of genes or proteins. We propose here a mutational mechanism to explain this predominance of deletions.