Na(+)-coupled HCO(3)(-) transporters (NCBTs) of the SLC4 family play critical roles in pH regulation as well as transepithelial HCO(3)(-) transport. We systematically examined, in the mouse reproductive tract tissues, the mRNA expression of five NCBTs as well as the five NBCe1 (Slc4a4) variants NBCe1-A through -E, of which NBCe1-D and NBCe1-E are novel. Cloning of NBCe1-D and NBCe1-E, both lacking a 27-nucleotide cassette I, reveals a novel alternative splicing unit in the mouse Slc4a4 gene. Transcripts of Slc4a4 lacking cassette I are expressed in diverse murine tissues as shown by RT-PCR analysis and in diverse tissues of other vertebrate species as shown by blast against GenBank database. Genomic sequence analysis indicates that cassette I of SLC4A4 is conserved in all NCBT genes except for SLC4A5, which presumably lost cassette I during its evolution. Our present study represents an important step towards understanding the molecular physiology of NBCe1, and presumably other NCBTs. Copyright © 2011 Elsevier Inc. All rights reserved.