AbstractVarious forms of oncogenic ALK proteins have been identified in various types of human cancers. While Crizotinib, an ALK inhibitor, has been found to be therapeutically useful against a subset of ALK⁺ tumours, clinical resistance to this drug has been well recognized and the mechanism of this phenomenon is incompletely understood. Using the cellular thermal shift assay (CETSA), we measured the Crizotinib—ALK binding in a panel of ALK⁺ cell lines, and correlated the findings with the ALK structure and its interactions with specific binding proteins. The Crizotinib IC₅₀ significantly correlated with Crizotinib—ALK binding. The suboptimal Crizotinib—ALK binding in Crizotinib-resistant cells is not due to the cell-specific environment, since transfection of NPM-ALK into these cells revealed substantial Crizotinib—NPM-ALK binding. Interestingly, we found that the resistant cells expressed higher protein level of β-catenin and siRNA knockdown restored Crizotinib—ALK binding (correlated with a significant lowering of IC₅₀). Computational analysis of the crystal structures suggests that β-catenin exerts steric hindrance to the Crizotinib—ALK binding. In conclusion, the Crizotinib—ALK binding measurable by CETSA is useful in predicting Crizotinib sensitivity, and Crizotinib—ALK binding is in turn dictated by the structure of ALK and some of its binding partners.