Another type of mutation that can occur is a frameshift mutation. When a gene is copied, the action begins in the nucleus. There an mRNA strand copies the DNA strand exactly. It codes for a protein precisely, leaving no gaps or spaces separating the triplets. This set of connected triplets is called the reading frame. A frameshift mutation is caused by the addition or loss of a nucleotide, or nucleotides. This alters the content of every triplet codon that follows in a reading frame. Frameshift mutations usually result in a shortened abnormal or nonfunctional protein, and they can create an early STOP codon downstream. If the number of added or missing base pairs is a multiple of three, the resulting protein may be drastically altered, and its function will depend on the extent of these alterations.