ALK inhibitor therapy is individual cancer treatment, in which the targeted drug therapy is directed to a patient group that is likely to benefit from the therapy. The detection in the tumor of ALK gene (anaplastic lymphoma kinase) rearrangement is a prerequisite for the ALK inhibitor therapy for non-small cell lung carcinoma. The ALK assay should be performed for non-squamous cellular non-small cell lung carcinomas, especially adenocarcinomas. It is not recommended to be based on the patients' clinical features. The immunohistochemical method is well suited for screening of ALK positivity. The present recommendation is to ascertain the ALK gene rearrangement from immunopositive specimens by using the FISH procedure.