['Classical Hodgkin lymphoma (cHL) and primary mediastinal large B-cell lymphoma (MLBCL) share similar histologic, clinical, and genetic features. In recent studies, we found that disease-specific chromosome 9p24.1/JAK2 amplification increased JAK2 expression and activity in both cHL and MLBCL. This prompted us to assess the activity of a clinical grade JAK2 selective inhibitor, fedratinib (SAR302503/TG101348), in in vitro and in vivo model systems of cHL and MLBCL with defined JAK2 copy numbers.', 'We used functional and immunohistochemical analyses to investigate the preclinical activity of fedratinib and associated biomarkers in cell lines and murine xenograft models of cHL and MLBCL with known 9p24.1/JAK2 copy number.', 'Chemical JAK2 inhibition decreased the cellular proliferation of cHL and MLBCL cell lines and induced their apoptosis. There was an inverse correlation between 9p24.1/JAK2 copy number and the EC50 of fedratinib. Chemical JAK2 inhibition decreased phosphorylation of JAK2, STAT1, STAT3, and STAT6 and reduced the expression of additional downstream targets, including PD-L1, in a copy number-dependent manner. In murine xenograft models of cHL and MLBCL with 9p24.1/JAK2 amplification, chemical JAK2 inhibition significantly decreased JAK2/STAT signaling and tumor growth and prolonged survival. In in vitro and in vivo studies, pSTAT3 was an excellent biomarker of baseline JAK2 activity and the efficacy of chemical JAK2 inhibition.'] Does selective JAK2 inhibition specifically decrease Hodgkin lymphoma and mediastinal large B-cell lymphoma growth in vitro and in vivo?