Background: Preclinical studies demonstrated Brahma related gene 1 (BRG1) mutations or loss of expression, and mutations of LKB1 may be associated with lack of sensitivity for MEK inhibitor trametinib in a subset of KRAS mutant NSCLC lines. This study aimed to evaluate the frequency of KRAS, LKB1 and BRG1 mutations in NSCLC tumors; and determine whether KRAS mutations in corresponding plasma samples could be detected by evaluating circulating cell-free DNA (cfDNA). Methods: Human NSCLC FFPE tumor tissue and matched plasma samples were procured from Indivumed GmbH. KRAS mutation status of 101 NSCLC tumors and matched plasma were determined by direct sequencing of genomic DNA (gDNA) from tissue and/or BEAMing on tissue gDNA or plasma cfDNA. Genetic mutations of LKB1 and BRG1 were determined by direct sequencing. Additional mutations were determined using the Ion Torrent AmpliSeq Cancer Panel. BRG1 protein expression was evaluated by IHC. Results: By direct sequencing and BEAMing we found 27/101 (28.4%) NSCLC tissue and/or plasma samples harbored KRAS mutations: G12V (37.0%), G12C (29.6%), G12D (18.5%), G12S, G13C, G13D and Q61H (3.7% each). The KRAS mutation status concordance (mutant or wild-type) between tumor gDNA and plasma cfDNA was 79-81%. Among the KRAS mutant tumors, LKB1 and BRG1 mutations were detected in 10/26 (38%) and 1/26 (3.8%) tumors respectively by direct sequencing. By IHC, loss of BRG1 expression was detected in 1/21 KRAS mutant tumors. The mutation frequency and variants for KRAS and LKB1 in patient samples were comparable with KRAS mutant NSCLC cell lines and COSMIC database. However the frequency of BRG1 mutation and protein loss were much lower in patient tumors. In a subset of 15 KRAS mutant tumors, Ion Torrent confirmed KRAS and LKB1 mutations and provided additional mutations found in TP53, FGFR2, FGFR3, GNAS, KDR, KIT and MET. Conclusions: This study demonstrates that KRAS mutant NSCLC tissues have high frequency of LKB1 mutations along with other mutations. It also supports the feasibility of detection of KRAS mutations in cfDNA from blood of NSCLC patients using BEAMing technology, providing an alternative to invasive biopsy.