Figure 1. (a) Crystal structure of interleukin 3 (Feng, 1995), an example of a 4- helical cytokine with short chains. (b) Crystal structure of interleukin 6 (Somers et al., 1997), a long-chain 4- helical cytokine.

Figure 2. Crystal structure of the growth hormone receptor complex (De Vos et al., 1992), a prototype for class I haematopoietic growth factor receptors. Shown in red, green, yellow and blue are the four helices of growth hormone, which is in contact with two identical growth hormone receptor subunits (coloured magenta and cyan).

Figure 3. Signal transduction pathways for granulocyte–macrophage colony-stimulating factor (GM-CSF). Signalling occurs after receptor oligomerization (_c and GMR subunits) in the presence of GM-CSF. Kinases such as JAK2 are activated and phosphorylate tyrosine (Y) residues present on the receptor cytoplasmic tail and other signalling molecules (STAT, SHIP (Src homology 2 -containing inositol phosphatase), Shc, SHP-2 (Src homology 2 -containing phosphatase)). This leads to activation of ras and contributes to cell proliferation via Raf^–1 and MAPK (mitogen activated protein kinase). Cell survival is mediated in part by activation of PI3 kinase and the serine kinase Akt. The pathways responsible for differentiation are largely unknown. Other cytokines which use class I haematopoietic receptors have similar signalling mechanisms.