This study identifies and validates fully chemically-modified small interfering RNAs (siRNAs) that are cross-reactive to Janus kinase 1 (JAK1) in humans and a wide variety of other species. These siRNAs were tested in cell lines and skin tissues of multiple species, demonstrating their ability to silence JAK1. JAK1 is a key member of the JAK family of tyrosine kinases involved in inflammatory cytokine signaling pathways. Dysregulation of these pathways is often involved in the pathogenesis of immune disorders, and modulation of JAK family enzymes is an effective strategy in the clinic. The study presents strategies for identifying and validating siRNAs with cross-species activity, focusing on JAK1-targeting siRNAs that have broad cross-species targetability. The researchers identified and optimized several JAK1-targeting siRNAs, including si883, which showed cross-species activity in human, pig, and other species. They also identified si860, which showed efficient silencing of JAK1 in multiple species. The study highlights the potential of these siRNAs for evaluating the modulation of JAK1 in animal models of human inflammatory skin diseases. The findings suggest that targeting conserved sites of mRNAs across multiple species is a viable strategy for identifying cross-species reactive compounds for preclinical development. The study also discusses the importance of chemical modifications in enhancing the activity and stability of siRNAs, and the potential for further chemical engineering to expand the utility of these compounds for treating immune disorders in additional tissues. The study was supported by Aldena Therapeutics and the National Institutes of Health. The authors include contributions from multiple institutions and have declared interests related to the research.This study identifies and validates fully chemically-modified small interfering RNAs (siRNAs) that are cross-reactive to Janus kinase 1 (JAK1) in humans and a wide variety of other species. These siRNAs were tested in cell lines and skin tissues of multiple species, demonstrating their ability to silence JAK1. JAK1 is a key member of the JAK family of tyrosine kinases involved in inflammatory cytokine signaling pathways. Dysregulation of these pathways is often involved in the pathogenesis of immune disorders, and modulation of JAK family enzymes is an effective strategy in the clinic. The study presents strategies for identifying and validating siRNAs with cross-species activity, focusing on JAK1-targeting siRNAs that have broad cross-species targetability. The researchers identified and optimized several JAK1-targeting siRNAs, including si883, which showed cross-species activity in human, pig, and other species. They also identified si860, which showed efficient silencing of JAK1 in multiple species. The study highlights the potential of these siRNAs for evaluating the modulation of JAK1 in animal models of human inflammatory skin diseases. The findings suggest that targeting conserved sites of mRNAs across multiple species is a viable strategy for identifying cross-species reactive compounds for preclinical development. The study also discusses the importance of chemical modifications in enhancing the activity and stability of siRNAs, and the potential for further chemical engineering to expand the utility of these compounds for treating immune disorders in additional tissues. The study was supported by Aldena Therapeutics and the National Institutes of Health. The authors include contributions from multiple institutions and have declared interests related to the research.