Deletion mutation. Definition. noun. A type of gene mutation wherein the deletion (as well as addition) of (a number of) nucleotide (s) causes a shift in the reading frame of the codons in the mRNA, thus, may eventually lead to the alteration in the amino acid sequence at protein translation.Supplement. type of gene mutation wherein the deletion (as well as addition) of (a number of) nucleotide (s) causes a shift in the reading frame of the codons in the mRNA, thus, may eventually lead to the alteration in the amino acid sequence at protein translation. Supplement.