When an oligonucleotide is too close to the end of its target sequence, it may not be possible to have 15 nucleotides available. In such case, the available sequence is extended to 15 nucleotides by using either a poly A on the 3' side or a poly T on the 5' side. The real sequence is provided in lower case and the artificial tail in upper case. Here is an example of an oligonucleotide close to the 3' end of its target sequence and with 15 nucleotides added to its 3' end :