LAUSR

The metabolism of nucleic acids, such as DNA and RNA, involves their synthesis and degradation through chemical modifications of their constitutive units, the nucleotides. Nucleotide synthesis generally involves chemical reactions that add a phosphate, pentose sugar, and a nitrogenous base or nucleobase, while their degradation may involve the disassembly of the nucleotide to recycle its parts, or its modification to be transformed into another molecule. Both synthesis and degradation require separate enzymes to perform the chemical reactions, and each enzyme may have very restricted targets and operate in specific cell types throughout the body. One of these factors is the cytoplasmic enzyme thymidine phosphorylase (TP), which catalyzes the removal of the pentose sugar from the pyrimidine nucleosides thymidine and deoxyuridine, converting them to thymine or uracil, thus regulating the levels of the thymidine nucleoside in cells [1]. In humans, TP is coded by the TYMP gene, which maps to 22q13.33 and encodes a protein of approximately 45-kD in weight [1]. Despite its expression in most human tissues, the thymidine phosphorylase enzyme was originally identified in platelets and named “plateletderived endothelial cell growth factor” and was associated with promoting angiogenesis in vivo and stimulating in vitro cell growth specifically in endothelial cells [1,2]. In their exhaustive and comprehensive review article, Drs. Li and Yue have escorted the reader through a long and fascinating journey uncovering the many roles of the TYMP gene in the metabolism of nucleic acids and how genetic defects are associated with human diseases [1]. In human genetics, homozygous or compound heterozygous pathogenic variants in the TYMP gene have been almost exclusively