A huge variety of different intracellular signalling events initiated by either the extracellular environment or intracellular conditions is operative in intact living cells. For the sake of simplicity, these pathways are usually considered as almost separate entities. Some light has recently been shed upon a cross point of two possibly interconnected pathways. One is initiated by activated trimeric GTP-binding proteins and the other through receptors that have intrinsic tyrosine kinase activity upon stimulation. The cross point is thought to be the tyrosine kinase PYK2. I am going to investigate this kinase and its partners in a model system where I shall take advantage of the diversity of recently characterized different PC12 clones. These PC12 clones are quite diverse in terms of expression of a variety of marker proteins as well as in behaviour towards extracellular stimuli, thus providing an interesting tool to dissect the intracellular signalling pathways acting in these clones. Activity determination of various kinases, phosphatases, transcription factors and genes will be established and in a further step all these activities will be compromised and the reactions of the so treated clones towards extracellular stimuli will be investigated. With this approach the putative links between the two signalling pathways should become more clearer.