GCM proteins constitute a small transcription factor family. Nuclear localization of Drosophila GCM is mediated by a typical bipartite nuclear localization sequence (NLS) close to the DNA-binding GCM domain. Here, we have analyzed nuclear localization of the mammalian GCM proteins. Whereas GCMb/Gcm-2 contained a classical bipartite NLS, nuclear localization of GCMa/Gcm-1 was mediated by two regions without resemblance to known NLS, one corresponding to the amino-terminal part of the GCM domain, the second defined as a tyrosine-and-proline-rich carboxy-terminal region. Nuclear import was counteracted by an amino-terminal nuclear export activity. This complex regulation of subcellular localization has important implications for GCMa/Gcm-1 function.