The EGR1 (early growth response 1) gene is located in the chromosomal region 5q31.2. Deletions spanning the region 5q31.2 are among the most common reoccurring abnormalities detectable in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). The EGR1 protein belongs to the EGR family of C2H2-type zinc-finger proteins. It is a nuclear protein and functions as a transcriptional regulator. Deletion of EGR1 in estrogen receptor negative (ER-) breast carcinomas is correlated with a higher tumor grade, suggesting that loss of the EGR1 gene (and thereby loss of functioning EGR1 protein) may contribute to the pathogenesis of ER breast carcinomas. In patients with therapy-related MDS and AML, dicentric chromosomes have often been observed. In such conditions, many patients show a complex karyotype with several marker chromosomes unidentifiable by conventional cytogenetics. Fluorescence in situ Hybridization (FISH) has now made the characterization of these rearrangements much easier.