High mobility group A (HMGA) proteins are chromatinic proteins that do not have transcriptional activity per se, however, by interacting with the transcription machinery, they regulate, negatively or positively, the expression of several genes. We searched for genes regulated by HMGA1 proteins using microarray analysis in embryonic stem (ES) cells bearing one or two disrupted hmga1 alleles. We identified 87 transcripts increased and 163 transcripts decreased of at least 4-fold in hmga1-/- ES cells. For some of them, a HMGA1-dose dependency was observed, because an intermediate level was observed in the heterozygous ES cells. When the expression analysis of these genes was extended to embryonic fibroblasts and adult tissues such as heart, spleen, and liver from hmga1-knockout mice, contrasting results were obtained. In fact, aside some genes showing the same HMGA1 regulation observed in ES cells, there were some genes that did not modify their expression, and others showing a HMGA1-mediated regulation but in an opposite direction. These results clearly indicate that HMGA1-mediated gene regulation depends on the cellular context. Finally for a couple of analyzed HMGA1-regulated genes, electrophoretic mobility shift assay and chromatin immunoprecipitation revealed a direct binding of HMGA1 proteins to their promoters, suggesting a HMGA1-direct regulation of their expression.