CTH and neoplasm: The ability of CTH to play a functional role in invading GBM cells was further supported by our finding from Ivy GAP analysis that pyridoxal kinase (PDXK), the enzyme that converts pyridoxine and other vitamin B6 precursors into pyridoxal-5′-phosphate (PLP), the bioactive form of CTH cofactor vitamin B6 (39), was enriched at the leading edge of the tumor relative to the core (P < 0.001; Supplemental Figure 4H).