Age of disease onset in Wilson disease patients grouped according to their type of ATP7B mutation. Analysis of Wilson disease (WD) patients possessing either (1) two 'severe' mutations (SM/SM); (2) one 'severe' mutation and one missense mutation (SM/MM), or (3) two missense mutations (MM/MM). Distribution of data presented as box and whisker plots: 25th percentile, median, 75th percentile, maximum, outliers (circles), and extreme values (x). Both assays resulted in no indeterminate results or missing data. a, difference for the comparison with SM/MM patients; b, difference for the comparison with MM/MM patients. Age of onset is significantly earlier in SM/SM patients (13 years; IQR, 9 - 13) compared to SM/MM patients (16 years; IQR, 14 - 18) and MM/MM patients (22 years; IQR, 14 - 27).