The three known members of the HMGI(Y) family of high-mobility group (HMG) mammalian nonhistone nuclear proteins (HMG-I, HMG-Y, and HMGI-C) are thought to participate in numerous biological processes (transcription, replication, retroviral integration, genetic recombination, etc.) by virtue of their ability to recognize and alter the structure of both DNA and chromatin substrates. In vitro and in vivo the HMGI(Y) proteins preferentially bind to the narrow minor groove of stretches of AT-rich DNA by means of highly conserved peptide motifs called AT hooks. In vitro the HMGI(Y) proteins also have the ability to selectively bind to distorted DNA structures and to bend, unwind, and supercoil DNA substrates. Additionally, the HMGI(Y) proteins have the ability to interact with various protein transcription factors both in vitro and in vivo. Specific protein-DNA and protein-protein interactions permit the HMGI(Y) proteins to function as architectural transcription factors that regulate gene expression in vivo by controlling the formation of stereospecific multiprotein complexes on the AT-rich regions of certain gene promoters. Transcriptional overexpression of the HMGI(Y) genes is highly correlated with both cancerous transformation and increased metastatic potential of a number of different cancers, and chromosomal rearrangements involving AT-hook motifs have been associated with various types of benign human mesenchymal tumors. The levels of HMGI(Y) proteins in human cells have been proposed to be sensitive diagnostic indicators of both neoplastic transformation and metastatic progression. Drugs based on the AT-hook motif offer the potential for development of new tumor therapeutic reagents.