The highly complex pattern of expression of major histocompatibility complex class II (MHC‐II) molecules determines both the immune repertoire during development and subsequently the triggering and the control of immune responses. These distinct functions result from cell type‐restricted expression, developmental control and either constitutive or inducible expression of MHC‐II genes. Yet, in these various situations, MHC‐II gene expression is always under the control of a unique transactivator, CIITA. Here we show that the CIITA gene is controlled by several distinct promoters, two of which direct specific constitutive expression in dendritic cells and B lymphocytes respectively, while another mediates γ‐interferon‐induced expression. Thus the cellular, temporal and functional diversity of MHC‐II expression is ultimately controlled by differential activation of different promoters of a single transactivator gene. This provides novel experimental tools to dissect compartment‐specific gain or loss of MHC‐II function in vivo.