There are four sequence elements within introns of mammalian pre-mRNAs which are required for efficient splicing. These are the conserved sequence at the 5′ splice junction, the invariant AG dinucleotide at the 3′ splice junction, a weakly conserved sequence at the site of lariat formation etc.. Cryptic splice sites are also such kind of motif that are present in many regions throughout pre-mRNAs and yet are not recognized under normal conditions,whereas in transcripts bearing point mutations or deletions within natural elements,such sites can be activated^[1].