deletion

A deletion is a mutation that removes one or more nucleotides from a DNA sequence, the opposite of an insertion. Letters are dropped from the genetic text, and what remains closes the gap, leaving the sequence shorter than before.

Deletions span an enormous size range. At one extreme a single base is lost; at the other, thousands or millions of bases — sometimes a whole gene or a visible piece of a chromosome — can disappear. Large chromosomal deletions are studied in cytogenetics, while tiny deletions are detected by DNA sequencing.

As with insertions, a deletion within a coding sequence may or may not shift the reading frame depending on whether the number of bases removed is a multiple of three. Losing three bases trims one amino acid; losing one or two scrambles every codon that follows. The deletion of phenylalanine codon F508 in the CFTR gene, the most common cause of cystic fibrosis, removes exactly three bases and so does not shift the frame, yet still cripples the protein.