TYMS and colorectal cancer: TS promoter enhancer region (TSER) 2R/3R repeat polymorphisms reflect double (2R) or triple (3R) repeats of a 28-bp sequence upstream of the TS translational start site.[10] Metastatic colorectal and gastric cancers homozygous for the triple repeat (3R/3R) have higher TS expression than those homozygous for the double repeat (2R/R).[11, 12] Poorer response to 5-FU occurs in patients with the 3R allele in colorectal cancer.[13] In addition, a G to C single nucleotide polymorphism (SNP) in the second repeat of the 3R allele has been identified.