Regulation, recognition and cell signaling involve the coordinated actions of many players. To achieve this coordination, each participant must have a valid identification (ID) that is easily recognized by the others. For proteins, these IDs are often within intrinsically disordered (also ID) regions. The functions of a set of well-characterized ID regions from a diversity of proteins are presented herein to support this view. These examples include both more recently described signaling proteins, such as p53, α-synuclein, HMGA, the Rieske protein, estrogen receptor α, chaperones, GCN4, Arf, Hdm2, FlgM, measles virus nucleoprotein, RNase E, glycogen synthase kinase 3β,ℓ p21Waf1/Cip1/Sdi1, caldesmon, calmodulin, BRCA1 and several other intriguing proteins, as well as historical prototypes for signaling, regulation, control and molecular recognition, such as the lac repressor, the voltage gated potassium channel, RNA polymerase and the S15 peptide associating with the RNA polymerase S-protein. The frequent occurrence and the common use of ID regions in important protein functions raise the possibility that the relationship between amino acid sequence, disordered ensemble and function might be the dominant paradigm for the molecular recognition that serves as the basis for signaling and regulation by protein molecules. Copyright © 2005 John Wiley & Sons, Ltd.