We report the case of a 1-year-old male patient with systemic juvenile
xanthogranuloma in whom central nervous system lesions caused
developmental retardation, spasticity and clonus of the lower
extremities. He needed tube feeding and experienced severe bronchitis
and generalised convulsive seizure. KIF5B-ALK fusion was identified in
the cutaneous lesion, and he was administered alectinib, an ALK
inhibitor. Two months after the initiation of alectinib administration,
the central nervous system lesions achieved partial regression. The
spasticity and clonus were also relieved. A high index of cognition is
needed for ALK fusion in infants with histiocytosis.