Results: High plasma tyrosine concentrations were frequently documented in 7 out of 9 patients. Psychometric testing revealed a total IQ score below the average and an inhomogenous test profile with significant differences between the different testing scales in 5 out of 7 patients, respectively. Motor abilities were subnormal in 4 out of 7 patients. Logopedic evaluation in children at school age documented dysfunction or retardation in language development in all but one of the tested patients, however, all but one patient had a migration background.