ALK translocation is the 3rd most frequent genetic aberration in lung adenocarcinoma, and several inhibitors are now clinically available in first and second line settings. Accordingly, molecular diagnostics of ALK-positive lung cancer is very important and can be done with the rational combination of several methods. All international recommendations suggest that, except for cytological samples, screening technology for ALK-positive tumors is immunohistochemistry using a validated test. It is highly recommended that in case of ALK protein positive samples gene translocation must be confirmed by fluorescent in situ hybridization (FISH). In case of cytological samples FISH technique must be used as ALK diagnostics. In equivocal cases the genetic alteration of ALK can be confirmed by alternative molecular techniques such as next generation sequencing or RNAbased PCR methods. Upon administration of ALK inhibitors, acquired resistance is frequent which is mostly due to ALK amplification and/or mutation. It is evident that the diagnostics of these secondary ALK gene alterations must be done from recurrent tumors or circulating nucleic acids
