Since the ALK population tends to develop brain metastasis relatively often, roughly 30% at diagnosis, and knowing that crizotinib levels in the brain are less than in the blood or in other tissues [58], further ALK TKIs—namely, second- and third-generation ALK TKIs—have been developed in order to pass the brain barrier and to be effective even in the sanctuary of the brain. The gene discussed is ALK; the disease is metastasis.