Overview of repeats
X-linked spinal and bulbar muscular atrophy (SBMA), also known as the Kennedy disease is caused by CAG repeat expansions in the AR gene. The pathogenic repeat range in multiple studies is reported to be at least 40 (Atsuta et al., 2006; Doyu et al., 1993; Fratta et al., 2014; La Spada et al., 1992; Spada et al., 1991). A patient with early onset of disease was found to have 68 repeat expansion, which is the known largest so far (Grunseich et al., 2014). In 75 control samples the range was determined to be between 17 to 26 repeats (Spada et al., 1991). A study conducted later in Finnish population reported the normal allele length to be up to 35 repeats (Udd et al., 1998).