The mouse aspartyl beta-hydroxylase gene (Asph, BAH) has been cloned and 
characterized. The mouse BAH gene spans 200 kilobase pairs of genomic DNA and 
contains 24 exons. Of three major BAH-related transcripts, the two largest 
(6,629 and 4,419 base pairs) encode full-length protein and differ only in the 
use of alternative polyadenylation signals. The smallest BAH-related transcript 
(2,789 base pairs) uses an alternative 3' terminal exon, resulting in a protein 
lacking a catalytic domain. Evolutionary conservation of this noncatalytic 
isoform of BAH (humbug) is demonstrated in mouse, man, and Drosophila. 
Monoclonal antibody reagents were generated, epitope-mapped, and used to 
definitively correlate RNA bands on Northern blots with protein species on 
Western blots. The gene for mouse junctin, a calsequestrin-binding protein, was 
cloned and characterized and shown to be encoded from the same locus. When 
expressed in heart tissue, BAH/humbug preferably use the first exon and often 
the fourth exon of junctin while preserving the reading frame. Thus, three 
individual genes share common exons and open reading frames and use separate 
promoters to achieve differential expression, splicing, and function in a 
variety of tissues. This unusual form of exon sharing suggests that the 
functions of junctin, BAH, and humbug may be linked.