Signalling through receptors for cytokines and numerous growth factors activates members of the Janus kinase (also comically known as `just another kinase' or more simply JAK) family. These in turn switch on signal transducers and activators of transcription (STATs), which regulate transcription of various target genes. There are at least four JAKs and seven STATs in mammals - not to mention an array of endogenous inhibitors termed suppressors of cytokine signalling (SOCS). In Cell Science at a Glance (see p. 1281 + poster), Douglas Harrison and co-workers survey the JAK/STAT signalling pathway, detailing the various points at which crosstalk with other signalling mechanisms occurs, as well as the consequences of mutations in particular JAK and STAT genes.