To date, ALK fusion partners identified in IMT are ATIC (5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase), CARS (cysteinyl-tRNA synthetase), TPM3 (tropomyosin 3), TPM4 (tropomyosin 4), CLTC (clathrin heavy chain), RANBP2 (Ran binding protein 2), SEC31L1 (SEC31 homologue A (Saccharomyces cerevisiae), and PPFIBP1 (protein-tyrosine phosphatase, receptor-type, F polypeptide-interacting protein-binding protein 1). Immunohistochemistry (IHC) is simple, cost-effective, and theoretically useful in detecting ALK-positive tumors with any (including unknown) fusion partner. The concern about IHC is the inability of the conventional polymer method to detect the ALK fusion protein because of its low expression level. The intercalated antibody-enhanced polymer (iAEP) method could overcome this issue. Two ALK-negative IMT cases by conventional IHC have been proven to be ALK-positive by the iAEP method and finally revealed to have a new fusion partner, PPFIBP1. Thus, reassessment by highly sensitive IHC is likely to yield more ALK-positive cases in ALK-negative cases defined by conventional IHC. Break-apart fluorescence in situ hybridization (FISH) for ALK is also useful to confirm the separation ALK locus. However, it is inferior to IHC in terms of cost-effectiveness. Therefore, the currently most reliable diagnostic method is to detect ALK-positive IMT by highly sensitive IHC first and then confirm the translocation by break-apart FISH.