We have identified splicing variants of the mouse a4 subunit which have the same open reading frame but have a different 5'-noncoding sequence. Further determination of the 5'-upstream region of the a4 gene in mouse indicated the presence of two first exons (exon 1a and exon 1b) which include the 5'-noncoding sequence of each variant. The mRNAs of both splicing variants (a4-I and a4-II) show a similar expression pattern in mouse kidney by in situ hybridization. However, tissue and developmental expression patterns of the variants are different. In addition to strong expression in kidney, a4-I expression was detected in heart, lung, skeletal muscle, and testis, whereas a4-II is expressed in lung, liver, and testis. During development, a4-I was expressed beginning with the early embryonic stage, but a4-II mRNA was detected from day 17. These results suggest that each a4 variant has both a tissue and developmental stage specific function.