Three CTNNB1 wild-type MBs harboured pathogenic APC germline variants, explaining the WNT pathway activation seen in these patients and underscoring the need to perform genetic testing for APC carrier status (that is, Turcot syndrome)25 when WNT MB is suspected despite failure to detect mutant CTNNB1. Monosomy 6, a signature chromosomal alteration characteristic of patients with WNT MB, was confirmed in 83% of cases (Extended Data Figs 6a, 7a–c), demonstrating that neither CTNNB1 mutation nor chromosome 6 loss are universally present in all patients with WNT MB. Here, CTNNB1 is linked to Non-polyposis Turcot syndrome.