We have isolated, in guinea-pig endometrial cells, an estrogen-induced 1.8 kb RNA called gec1, Screening of a guinea-pig genomic library led to identification of gec1 gene consisting of 4 exons and 3 introns. Exon 1 contains the 5 ' UTR and the ATG initiation codon. A guinea-pig gec1 cDNA was obtained by 5 ' -RACE. The 351 bp coding sequence shares 76.8% identity with that of the human GABARAP 924 bp cDNA while UTRs of the two cDNAs differ. A gec1 probe from the 3 ' UTR revealed a 1.9 kb mRNA in human tissues and a human GEC1 cDNA was isolated from placenta. Its coding sequence shares 93 and 79% identity with that of guinea-pig gec1 and human GABARAP, respectively. The human and guinea-pig GEC1 proteins have 100% identity. GEC1 and GABARAP proteins have 87% identity and N terminus featuring a tubulin binding motif. Thus, estrogen-regulated gec1 is a new gene which could encode a microtubule-associated protein.