A case report of atypical Spitz tumor harboring a novel MLPH-ALK gene fusion with discordant ALK immunohistochemistry results.

Frequent kinase fusions have been reported in spitzoid neoplasms, approximately 10% of which involve ALK rearrangements. Herein, we report a case of atypical Spitz tumour (AST) with a novel MLPH-ALK fusion, which has not been previously reported to contribute to cancer development. The tumour was detected in the right arm of a 40-year-old woman. The novel ALK fusion was identified by a 5'-rapid amplification of cDNA ends-based system optimised for formalin-fixed paraffin-embedded tissue. Initially, ALK expression was detected by immunohistochemistry (IHC) using 5A4 antibodies for both sensitive and conventional polymer detection method. However, the anti-ALK1 antibody, which is commonly used for the diagnosis of ALK-positive anaplastic large cell lymphoma, failed to confirm ALK expression. These results indicated that ALK IHC results in ALK-rearranged AST may differ based on the type of primary antibody clone, which can be a potential diagnostic pitfall.