The genomic structure, transcription start site, polyadenylation signals, and promoter of the human Na+/ myo-inositol cotransporter (SLC5A3) gene have been elucidated through cloning, sequencing, mRNA analyses, and reporter gene assays. The gene consists of one promoter and two exons spanning approximately 26 kb. Exon 1 contains 175 bp of 5' untranslated sequence and is 15 kb upstream of exon 2. The 9.5-kb exon 2 contains the entire 2157-bp open reading frame and a large 3' untranslated sequence with seven putative polyadenylation signals. Multiple messages with different-sized 3' untranslated regions can be detected on Northern blots. Hypertonic stress caused mRNA levels, and primarily that of the full-length 9.5-kb transcript, to increase in cultured melanoma cells; ribonuclease protection analysis demonstrated that the transcription start site was the same in stressed as in control cells. The SLC5A3 gene functions in cellular osmoregulation and is expressed in many human tissues including the brain, kidney, and placenta. It is localized to chromosome 21q22.1. An overexpression of the SLC5A3 gene deserves consideration as a factor in the pathophysiology of Down syndrome.