11Improvement of feed efficiency (FE) is key for sustainability and cost reduction in pig 12 production. Our aim was to characterize the muscle transcriptomic profiles in Danbred 13Duroc (Duroc) and Danbred Landrace (Landrace), in relation to FE for identifying 14 potential biomarkers. RNA-seq data was analyzed employing differential gene expression 15 methods, gene-gene interaction and network analysis, including pathway and functional 16 analysis. We compared the results with genome regulation in human exercise data. In the 17 differential expression analysis, 13 genes were differentially expressed, including: 18 MRPS11, MTRF1, TRIM63, MGAT4A, KLH30. Based on a novel gene selection method, 19the divergent count, we performed pathway enrichment analysis. We found 5 significantly 20 enriched pathways related to feed conversion ratio (FCR). These pathways were mainly 21 mitochondrial, and summarized in the mitochondrial translation elongation (MTR) 22pathway. In the gene interaction analysis, highlights include the mitochondrial genes: 23 PPIF, MRPL35, NDUFS4and the fat metabolism and obesity genes: AACS, SMPDL3B, 24 CTNNBL1, NDUFS4 and LIMD2. In the network analysis, we identified two modules 25 significantly correlated with FCR. Pathway enrichment of modules identified MTR, 26 electron transport chain and DNA repair as enriched pathways. In the network analysis, 27 the mitochondrial gene group NDUF was a key hub group, showing potential as 28 biomarkers. Comparing with human transcriptomic exercise studies, genes related to 29 exercise displayed enrichment in our FCR related genes. We conclude that mitochondrial 30 activity is a driver for FCR in muscle tissue, and mitochondrial genes could be potential 31 biomarkers for FCR in pigs. We hypothesize that increased FE mimics processes 32 triggered in exercised muscle. 33 34 35 calculation is dependent on individual population and production factors (Hoque, Kadowaki et al. 56 2009, Do, Strathe et al. 2013. However, in pig production, the side effects of FCR selection and 57 simplicity are desired traits, thus perhaps explaining why the pig population in Denmark and in 58 general pig production, FCR has been the main efficiency phenotype used for selection (Gilbert, 59 Billon et al. 2017). One can also hypothesize that FCR is more easily translatable between 60 breeds/populations, as it is a simple dimensionless ratio, which has a simple and generally 61 comparable interpretation. In contrast, it is more difficult to easily compare RFI values across 62 different populations or breeds. In regards to the biological and/or genetic background of FCR in 63 pigs, the results remain somewhat elusive (Ding, Yang et al. 2018), thus inviting for further analysis 64 on the topic. 65