HAAO and central nervous system cancer: As for the genes of the “hallmarks”, we found that 3-HAO is the second enzyme with more positive correlations in the gene set studies here, but it is of note that the lack of this enzyme is reported to non-neoplastic astrocytes and glioma cells, being the microglia of the principal cells where the 3-HAO is expressed to produce quinolinic acid, so then the glioma cells with an expression of QPRT can convert quinolinic acid to NAD+ [64].