The three different c-Jun N-terminal kinases (JNKs) are activated in multiple cell types by both apoptotic and mitogenic signals, and in turn regulate the activity of transcription factors such as c-Jun. Being highly homologous and ubiquitously expressed, the JNK1 and JNK2 proteins have been thought to perform redundant functions in many physiological process. However, our data from Jnk1-/- or Jnk2-/- cells and mice suggest that both JNK isozymes perform distinct functions in regulating cellular proliferation via differential regulation of c-Jun, which is a critical regulator of cell-cycle progression. Absence of JNK1, the positive regulator of c-Jun, leads to decreased fibroblast proliferation. In contrast, JNK2 deficiency leads to reduced c-Jun degradation, thereby augmenting c-Jun levels and cellular proliferation. Various cell types including fibroblasts, erythroblasts and hepatocytes from Jnk2-/- mice exhibit increased proliferation rates compared to their wild-type counterparts. These data therefore suggests that JNK2, in contrast to JNK1, is a negative regulator of cellular proliferation in multiple cell types.