SCA27B is part of a group of genetic conditions known as REPEAT EXPANSION DISORDERS. 

Recall from the "What is a gene?" page that the DNA code that our genes are written in is made up of four letters: A, G, C, and T. In one part of the FGF14 gene, the three letters "GAA" are repeated over and over. This part of the FGF14 gene is known as the GAA REPEAT. The number of times that "GAA" is repeated in the FGF14 gene varies from person to person. Also recall that each person has 2 copies of the FGF14 gene.

To understand SCA27B, it is important to understand the concept of "penetrance." PENETRANCE is the amount of people with a particular genetic variant who will actually have signs and symptoms of the genetic condition associated with the gene.

• When nearly everyone with a pathogenic (disease-causing) variant in a gene develops the disease, the variant is FULLY PENETRANT

• When some people with a pathogenic (disease-causing) variant in a gene develop the disease but some people with the variant do not, the variant is INCOMPLETELY PENETRANT.  

People who do not have SCA27B usually have between 8 to 300 GAA repeats in both copies of FGF14. 

People who have a copy of FGF14 with 250 to 300 GAA repeats may or may not develop SCA27B. The 250 to 300 repeat FGF14 variant is INCOMPLETELY PENETRANT.  

People who have a copy of FGF14 with more than 300 GAA repeats almost always will develop SCA27B at some point in their lives. The 300+ repeat FGF14 variant is FULLY PENETRANT.4,5