To function correctly, each cell depends on thousands of proteins to do their jobs in the right places at the right times. Sometimes, gene variants (also known as mutations) prevent one or more proteins from working properly. By changing a gene’s instructions for making a protein, a variant can cause a protein to malfunction or to not be produced at all. When a variant alters a protein that plays a critical role in the body, it can disrupt normal development or cause a health condition. A condition caused by variants in one or more genes is called a genetic disorder.

In some cases, gene variants are so severe that they prevent an embryo from surviving until birth. These changes occur in genes that are essential for development, and often disrupt the development of an embryo in its earliest stages. Because these variants have very serious effects, they are incompatible with life.

It is important to note that genes themselves do not cause disease—genetic disorders are caused by variants that alter or eliminate a gene’s function. For example, when people say that someone has “the cystic fibrosis gene,” they are usually referring to a version of the CFTR gene that contains a variant that causes the disease. All people, including those without cystic fibrosis, have a version of the CFTR gene.