An identical GATA1 mutation in twins with TMD accompanying DS. (A) Direct sequence analysis of cDNA and genomic DNA from TMD blast cells of identical twins with DS showed that 20 nucleotides of the duplicated sequences were inserted in exon 2 of the GATA1 gene in both patients. (B-C) The mutation resulted in the introduction of a premature stop codon in the gene sequence encoding the N-terminal activation domain. The predicted protein, GATA-1s, lacks the transactivation domain.