UTR or coding region?

Hi, everyone,
I am studying a T-DNA insertional mutant. It's very interesting that the
T-DNA heterozygous lines(Tt) are semi-sterile, but both the T-DNA homozygous
lines(TT) and the T-DNA negative lines(tt) are full fertile. the T-DNA is
inserted into the 3'UTR of the gene as indicated by KOME full-length cDNA
information. However, I found that the gene expression level is not changed
in both Tt or TT lines, so it seems that the protein structure may be
changed by the insertion and the semi-sterile is the consequence of
protein-protein interaction? So anyone can help me to judge the insertion
point is really in UTR or coding region? Thanks a lot!

Jian