Ago2 immunoprecipitation revealed miR-155 as the most abundant miRNA, and enrichment of target mRNAs C/EBPβ and SOCS1. To investigate its function, we over-expressed miR-155 in ALCL ALK+ cell lines and demonstrated reduced levels of C/EBPβ and SOCS1. In murine engraftment models of ALCL ALK−, we showed that anti-miR-155 mimics are able to reduce tumour growth. This evidence concerns the gene AGO2 and anaplastic large cell lymphoma.