The Janus kinase 1 (JAK1)/JAK2 inhibitor ruxolitinib, which has been shown to provide effective and lasting spleen size reduction and symptom mitigation as well as a survival advantage compared with placebo and best available therapy in randomized controlled clinical trials, has been approved for the treatment of myelofibrosis in more than 80 countries worldwide. This evidence concerns the gene JAK2 and myelofibrosis.