Mutations in the collagen gene can lead to the disease Osteogenesis Imperfecta, which is characterized by brittle bones. In the illustration, a stem-loop formed from an imperfect inverted repeat is mutated with a thymine (T) nucleotide insertion as a result of an inter- or intrastrand switch. The addition of the T creates a base-pairing "match up" with the adenine (A) that was previously a "bump" on the left side of the stem. While this addition makes the stem stronger and perfects the inverted repeat, it also creates a frameshift mutation in the nucleotide sequence which alters the reading frame and will result in an incorrect expression of the gene.