Background: Anaplastic lymphoma kinase (ALK) gene rearrangements define a distinct molecular subset of NSCLC. Recently, several studies have reported that ALK+ pts occasionally harbor concomitant mutations in other oncogenic drivers. Methods: We retrospectively analyzed tumor genotyping data from 1,683 pts with NSCLC seen at 3 U.S. centers from 2009 – 2012 to determine rates of overlapping alterations in EGFR, KRAS and ALK. Mutations in EGFR and KRAS were mainly identified using the SNaPshot multiplexed assay (>95% of cases). ALK FISH was performed in all cases. To determine if this prevalence is impacted by crizotinib, we also updated our earlier analysis (Katayama et al., Sci Transl Med, 2012) of a series of repeat biopsy specimens from 34 crizotinib-resistant, ALK+ pts. Resistant specimens were examined using ALK FISH, SNaPshot, and direct sequencing of the ALK tyrosine kinase domain (TKD). Results: Screening identified 301 (17.8%) EGFR mutations, 465 (27.6%) KRAS mutations, and 75 (4.4%) ALK rearrangements. EGFR mutations and ALK rearrangements were mutually exclusive. 4 pts with KRAS mutations also had abnormal ALK FISH patterns, involving isolated 5’ green probes (3/4 cases) and an isolated 3’ red probe that was unusually small (1/4 cases). Sufficient tissue was available for confirmatory ALK immunohistochemistry (clone 5A4, Novacastra, UK) in 3 of these cases, all of which were negative for ALK expression. Among pts with ALK+ NSCLC and acquired crizotinib resistance, repeat biopsy specimens remained ALK fusion positive in 28/28 (100%) cases. Secondary mutations in the ALK TKD (1151Tins, L1196M, G1202R, S1206Y, and G1269A) were identified in 7/34 (20.6%) cases. L1196M was the most common secondary mutation (3/34, 8.8% cases). ALK gene amplification was present in 3/28 (10.71%) pts. No EGFR or KRAS mutations were identified in 23 crizotinib-resistant, ALK+ pts with sufficient tissue for testing. Conclusions: Functional ALK rearrangements were mutually exclusive with EGFR and KRAS mutations in a large Western patient population. This lack of overlap was also observed in ALK+ pts with acquired resistance to crizotinib.