Figure 2. Analysis of UV-induced LOH for cells irradiated at G1. Cells that are heterozygous at a specific locus (“a” allele and “c” allele) are arrested at G1 and irradiated with UV (damage is represented by a star). Sectored colonies are selected after UV exposure.17 Each sector subcolony represents two sister chromatids. Therefore, the four chromatids are represented in the two sector colonies. Sequencing each sector can reveal which allele, “a” or “c,” was lost. If the number of sequence reads for “a” and “c” alleles is equal then heterozygosity is maintained. If there are no sequence reads for one of them, LOH has occurred. Through analysis of LOH in the two sectors it is possible to establish a 3:1 or 4:0 ratio for the four sister chromatids (see text and ref. 17).