The High Mobility Group A (HMGA) family is comprised of four proteins: HMGA1a, HMGA1b, HMGA1c and HMGA2 (formerly HMGI, HMGY, HMG-I/R and HMGI-C, respectively).They are encoded by two distinct genes,the HMGA 1 proteins being products of the same gene generated through alternative splicing.By interacting with the transcription machinery, HMGA1 proteins alter the chromatin structure and, thereby, regulate the transcriptional activity of several genes.They seem to play their major physiological role during embryonic development.HMGA protein expression has been found abundant in several malignant neoplasias, including pancreas, thyroid, colon, lung, ovary,prostate carcinomas.HMGA overexpression is mainly associated with a highly malignant phenotype, also representing a poor prognostic index since HMGA overexpression often correlates with the presence of metastasis, and with a reduced survival.The involvement of the HMGA proteins in breast cancer will be discussed.