These lessons from the IFN-era in CML are of utmost importance, since so many similarities exist between the CML-IFN-landscape and the MPN-IFN-landscape in regard to highly important questions such as “Why to treat with IFN-alpha2?” and “When to treat with IFN-alpha2?”. This evidence concerns the gene IFNA1 and chronic myelogenous leukemia, BCR-ABL1 positive.