Interleukin 4 (IL-4) is a cytokine that regulates growth and differentiation of lymphoid and nonlymphoid cells. To study the molecular basis of IL-4 function, we used a cDNA subtraction approach based on the representational difference analysis method. This subtractive amplification technique allowed us to use small amounts of RNA from lipopolysaccharide ± IL-4-stimulated normal B cells to obtain IL-4-induced genes from these cells. We report here on one such gene,Fig1(interleukin-four induced gene 1), the first characterized immediate–early IL-4 inducible gene from B cells.Fig1expression is strikingly limited to the lymphoid compartment. B cells, but not T cells or mast cells, expressFig1in response to IL-4 within 2 hr in a cycloheximide resistant manner. IL-2, IL-5, and Il-6 do not induceFig1in culture.Fig1maps betweenKlk1andLdh3on mouse chromosome 7, near two loci involved with murine lupus,Sle3andLbw5. TheFig1cDNA sequence encodes a predicted 70-kDa flavoprotein with best homology to the monoamine oxidases, particularly in domains responsible for FAD binding.