Citation: Cheng, C. M., Nair, A. D. S., Jaworski, D. C., & Ganta, R. R. (2015). Mutations in Ehrlichia chaffeensis Causing Polar Effects in Gene Expression and Differential Host Specificities. Plos One, 10(7), 13. doi:10.1371/journal.pone.0132657Ehrlichia chaffeensis, a tick-borne rickettsial, is responsible for human monocytic ehrlichiosis. In this study, we assessed E. chaffeensis insertion mutations impacting the transcription of genes near the insertion sites. We presented evidence that the mutations within the E. chaffeensis genome at four genomic locations cause polar effects in altering gene expressions. We also reported mutations causing attenuated growth in deer (the pathogen's reservoir host) and in dog (an incidental host), but not in its tick vector, Amblyomma americanum. This is the first study documenting insertion mutations in E. chaffeensis that cause polar effects in altering gene expression from the genes located upstream and downstream to insertion sites and the differential requirements of functionally active genes of the pathogen for its persistence in vertebrate and tick hosts. This study is important in furthering our knowledge on E. chaffeensis pathogenesis.