Structure of Haus3 locus. Shown are the following tracks from the UCSC browser (July 2007 NCBI37/mm9 assembly): Mouse RefSeq (red), Other Refseq (blue), and Mouse mRNAs (black). Refseq annotates Haus3 as being within the first intron of Poln, presumably based on the 2 "full length" mRNAs AK133063 and AY135562, which include a small 5' noncoding exon (far right). Interestingly, this exon was also found in a Haus3 full length mRNA as well as several spliced ESTs (not shown), but in only one spliced Poln EST (not shown). We posit that this 5' exon is not a normal part of Poln transcripts, but that occasional transcription initiated in that exon generally produced Haus3 mRNAs, but that occasionally there is readthrough to produce a Poln mRNA. Note that human, rat and cow Poln Refseq annotations do not include that exon, depicting the two genes as non-overlapping (with the exception of a potentially artifactually long 3' end of HAUS3 in the human). The L5Jcs31 mutation in Haus3 is indicated by an asterisk.