Specificity of signaling by hematopoietic cytokine receptors: Instructive versus permissive effects

The helical cytokines constitute a family of proteins with a common three-d imensional structure. They exert a wide variety of biological effects with a preference for the hematopoietic system, The effects of helical cytokines are mediated by cell surface receptors, which belong to the cytokine recep tor superfamily and signal by activating cytoplasmic tyrosine kinases of th e Janus kinase (Jak) family and other downstream signaling pathways. The re levance of each of these pathways for eliciting a specific cellular respons e remains to be determined. This review will focus on cytokine receptors wh ich play a role in the regulation of hematopoiesis and summarize data that address the question how specificity of signaling is achieved.