text: Paracrine signaling of growth factors between nearby cells has been shown to exacerbate carcinogenesis.  In fact, mutant forms of a single RTK may play a causal role in very different types of cancer.  The Kit proto-oncogene encodes a tyrosine kinase receptor whose ligand is a paracrine protein called stem cell factor (SCF), which is important in hematopoiesis (formation of cells in blood).  The Kit receptor and related tyrosine kinase receptors actually are inhibitory and effectively suppresses receptor firing.  Mutant forms of the Kit receptor, which fire constitutively in a ligand-independent fashion, are found in a diverse array of cancerous malignancies.