FIG. 1.

The YAC breakage assay. Cells containing YAC CF1, which contains a CAG/CTG tract and URA3 gene, are FOA^S. When breakage occurs inside the CTG tract (heavy black line), the broken YAC can be rescued by the addition of a new telomere to the backup telomere (C₄A₄ sequence). The resulting cells will be Leu2⁺ and FOA^R. The repeat tract is oriented such that the CAG sequence is on the lagging-strand template and the CTG sequence is on the Okazaki fragments. YAC CF1 is ∼62 kb long and contains 41 kb of lambda sequences between the LEU2 gene and the C₄A₄ sequence indicated by hatch marks. Drawing is not to scale.