Homozygous mutant mice are viable and appear normal, healthy, and fertile with no defects in lymphocyte development (thymocytes, splenocytes, and bone-marrow derived dendritic cells). Compared to control sibs, however, macrophages and primary embryonic fibroblasts from mutant mice show reduced activation of p38 MAPK. Cytokine IL-12 (encoded for by Il12a and Il12b) is strongly reduced in mutant macrophages and dendritic cells. Mutant fibroblasts have a defective response to Tnf. Mutant mice also have defective T-helper cells with respect to the ability to produce Ifng in response to antigen stimulation of keyhole limpet hemocyanin (KLH) both in vitro and in vivo (J:54078, J:54309).