Location of GAA/TTC tracts on chromosome V is depicted. Intact chromosome V is ~585 kb, DSB at the tract results in a 43 kb broken fragment. Chromosomal DNA was separated by CHEF. Both intact and broken chromosomal V were detected by southern blot hybridization using a HPA3-specific probe. Lanes: 1, wild-type strain with (GAA)20; 2, wild-type strain with (GAA)230; 3, TET-TAF4 strain with (GAA)230; 4, TET-RFA2 strain with (GAA)230.