AbstractAnaplastic lymphoma kinase (ALK) has emerged as an important oncogene in a number of human malignancies ranging from non‐Hodgkin lymphoma to neuroblastoma. In the former case, ALK is activated as a consequence of a chromosomal translocation and in the latter due to point mutations. In both cases the transforming potential of these oncogenic forms of ALK have been shown in vitro employing traditional cellular transformation assays including 3T3 foci formation. We reasoned that other ALK mutants which have been identified by the Cancer Genome Project may likewise possess transformation potential. We have selected seven ALK mutants identified in cell lines representative of a variety of human cancers based on position within the ALK protein, zygosity and frequency of detection including R1192Q, K1525E, C1021Y, R412C, A1252V, D1311A, K1518N and have compared their transformation capability in comparison to the published neuroblastoma‐associated F1174L ALK mutant when expressed in immortalized p53−/− murine embryonic fibroblasts. Whilst the F1174L mutant reproducibly drives foci formation in vitro, the other ALK mutants fail in this task. Furthermore, apart from the F1174L ALK mutant, the ALK protein is not phosphorylated on tyrosine residue 1604 suggesting that they are kinase‐inactive in this cellular context. We conclude that not all ALK mutants have transformation potential and may represent “passenger” mutations in the evolution of cancer. © 2011 Wiley Periodicals, Inc.