Ruxolitinib is a type of drug called a protein kinase inhibitor.
Genes called JAK1 and JAK2 help control the number of blood cells the body makes. If either of these genes become faulty, the body may make too many blood cells. Ruxolitinib blocks (inhibits) the signals that are sent by the faulty gene. This helps reduce the number of extra blood cells that are made.
Ruxolitinib can block other signals that cause too many blood cells to be made. So it can also work for people who do not have a faulty JAK gene.