 Thymidine phosphorylase is the enzymatic product of the TYMP gene and is responsible for breaking down thymidine nucleosides into thymine and 2-deoxyribose 1-phosphate.Without normal thymidine phosphorylase activity, thymidine nucleosides begin to build up in cells.High nucleoside levels are toxic to mitochondrial DNA and cause mutations that lead to dysfunction of the respiratory chain, and thus, inadequate energy production in the cells.