Activity of Janus kinase 2 (JAK2) in the JAK2/STAT5 signaling pathway is critically controlled by suppressor of cytokine signaling‐1 (SOCS‐1). We have previously shown that SOCS‐1 is biallelically mutated in the primary mediastinal B‐cell lymphoma (PMBL) cell line MedB‐1, resulting in impaired JAK2 degradation and sustained phospho‐JAK2 action. SOCS‐1 is frequently mutated in PMBL tumor primaries. Here, we report that the PMBL cell line Karpas1106P has a biallelic deletion of the SOCS‐1 region on chromosome 16p13.13. By fluorescence in situ hybridization and microsatellite analysis, this deletion was narrowed down to a range of 650 kb to 1.48 Mb. Like MedB‐1, Karpas1106P harbors gains of the JAK2 gene on chromosomal region 9p24 and elevated levels of JAK2 mRNA. Nevertheless, JAK2 protein was not increased but constitutively phosphorylated in Karpas1106P cells. In analogy to MedB‐1 cells, Karpas1106P cells exhibited a retarded degradation of de novo synthesized JAK2 protein revealed by pulse/chase experiments. Therefore, we conclude that loss of SOCS‐1 function either by mutation or by the complete deletion of the gene plays an important role in the dysregulation of JAK/STAT signaling in Karpas1106P and PMBL. © 2005 Wiley‐Liss, Inc.