In Wilson's disease (WD; OMIM# 277900), a discrepancy between the frequency of individuals carrying biallelic pathogenic variants in ATP7B (1:7000)20 and the prevalence determined by mass screening using ceruloplasmin in blood/urine (1–2:3000)21, 22 suggests the penetrance is not full, particularly for variants mapping outside a three‐exon hotspot gene region where 50% of WD‐causing mutations were identified in a UK population study.23 Here, ATP7B is linked to Wilson disease.