A (left side): Detection of TS 5'-UTR polymorphism. Thymidylate synthase (TS) 5'-UTR genotype analysis in matched normal (N) and tumor (T) DNA. The upper and lower bands represent PCR products from amplification of the TS segment containing 3R and 2R, respectively. Each patient has a heterozygous 2R/3R genotype in normal tissue, as indicated by the presence of both bands. Case 1: Loss of heterozygosity (LOH) gives rise to a tumor with a 2R/loss genotype. Case 2: LOH does not occur. Case 3: LOH gives rise to a tumor with a 3R/loss genotype. 1B (right side): Detection of TS 3'-UTR polymorphism. Thymidylate synthase (TS) 3'-UTR genotype analysis in matched normal(N) and tumor (T) DNA. The middle lane shows the double bands in 109 bp and 103 bp, representing the heterozygous 6 bp insertion/deletion. The right lane shows the single band in 109 bp, representing the loss of the 103 bp band.