A high-affinity, palindromic "double" GATA protein binding site in the Gata1 promoter presumed to mediate positive Gata1 autoregulation was replaced by a floxed Pgk-neo cassette; transient Cre recombinase expression in ES cells left a single loxP site flanked by two Not1 sites. The 21-bp deleted segment comprised nucleotides -691 through -671 upstream of the last nucleotide of the first hematopoietically expressed exon of Gata1. (J:88711)