TYW1 is a radical S-adenosylmethionine (SAM) enzyme that catalyzes wybutosine biosynthesis, a critical tRNA hypermodification pathway 1. Specifically, TYW1 catalyzes the second step of wybutosine synthesis by cyclizing N(1)-methylguanosine to form the tricyclic ring structure 2, utilizing two [4Fe-4S] clusters to generate radical intermediates 2. Wybutosine is a tricyclic hypermodified guanosine located at position 37 of eukaryotic phenylalanine tRNA, immediately 3'-adjacent to the anticodon, where it stabilizes codon-anticodon base-pairing during ribosomal decoding 32. Clinically, TYW1 loss-of-function mutations cause cerebral palsy with severe intellectual disability and primary microcephaly 4. Mechanistically, TYW1 deficiency blocks wybutosine (OHyW) formation in tRNAphe, reducing translation efficiency of UUU codons and impairing neuronal proliferation and migration 45. TYW1 loss specifically perturbs translation of proteins involved in cell cycling and activates HERVK retrotransposons through reduced SMARCAD1 expression, further impairing neural differentiation 45. Additionally, TYW1 variants have been associated with Behçet's disease susceptibility in genome-wide association studies 6 and identified as a potential driver gene in multiple myeloma 7.