Structure of NCBT proteins. (A) A generic structure of NCBTs is predicted to have an extended N-terminus, a transmembrane domain containing 14 transmembrane segments, and a relatively short C-terminal domain. The extracellular loop between segments 5 and 6 contains two N-glycosylation sites. (B) Alignment of protein sequence comprising human NCBTs. The alignment was performed using the UniProt (www.uniprot.org) with each canonical protein sequence for NBCe1 (Uniprot ID: Q9Y6R1), NBCe2 (Q9BY07), NBCn1 (Q9Y6M7), NBCn2 (Q2Y0W8), and NDCBE (Q6U841). Sequences highly conserved among NCBTs are shown in brown bars, while sequences moderately conserved are in open bars. Sequences with negligible homology are shown as a horizontal line. Internal splice cassettes are in different colors.