A transgenic non-human animal, such as a mouse, has a genome that include a nucleic acid construct having at least one transcriptional regulatory sequence capable of directing expression in B cells of the animal, wherein the transcriptional regulatory sequence is operably linked to a nucleic acid encoding a miR155 gene product. A method of testing the therapeutic efficacy of an agent in treating or preventing a lymphoproliferative condition includes assessing the effect(s) of the agent on a transgenic non-human animal.