JNK-interacting protein 1 (JIP-1) is an important scaffolding protein in the JNK signalling pathway. It is also believed to play a role in the mediation of mitogenic messages from the plasma membrane to the cell interior. Previous studies suggest that the JIP-gene is co-regulated with the insulin-like growth factor II (IGF II) gene, thereby contributing to the growth stimulatory effects of this potent growth factor. The striking coexpression of these two genes was found in murine fetuses as well as in primary human embryonic tumours. When six primary Wilms tumours were examined, the two genes showed a high degree of co-variation in the sense that high expression of IGF II was followed by high expression of JIP-1 and vice versa. However, when the human Wilms tumour cell line WCCS-1 was examined, a very modest intrinsic expression of IGF II was accompanied by a moderate expression of JIP-1. When exogenous IGF II was added, which has previously been shown to induce apoptosis in this cell line, the JIP-1 expression increased. These data suggest that JIP-1 has a more complex role in the regulation of proliferation as well as programmed cell death.