The gec1/GABARAPL1 (GABA(A)-receptor-associated protein like-1) gene has been identified as an early estrogen-regulated gene in guinea-pig cultured endometrial glandular epithelial cells (GEC). Guinea-pig and human gec1/GABARAPL1 proteins share 87% identity with GABARAP, which acts as a protein linker between microtubules and the GABA(A) receptor. To investigate the molecular mechanisms regulating gec1/GABARAPL1 gene expression, the 1.5-kbp region upstream of the translation initiation codon of the guinea-pig gec1/GABARAPL1 gene was cloned. A 300-bp fragment encompassing a pyrimidine-rich initiator element (INR) and the transcription start site (+1) was sufficient to initiate transcription. Transfection and gel shift experiments showed that a sequence located at +36/+50 in the first exon permitted induction of expression of this gene by estradiol acting via ERalpha. This sequence (GGGTCAACGTGACGT) differs only by one base pair from the consensus estrogen response element ERE (GGGTCAACGTGACCT). It can be concluded that the ERE located in the first exon encoding the 5'-untranslated region is sufficient for E2 activation of gec1/GABARAPL1 transcription.