None of the individuals with NBAS-induced (R)ALF carried two loss-of-function mutations in NBAS. However, NBAS mutations led to low levels of the gene product of about 25% NBAS protein (compared to healthy probands). The affected individuals recovered completely in the intervals between the crises and otherwise presented a rather mild phenotype, so reduced NBAS protein levels due to the mutations appear to be sufficient for normal function of most tissues including liver under afebrile conditions.