When an individual dies, the DNA degrades straight away. As time progresses the amount of meaningful DNA that scientists can work with diminishes. Oetzi's ancient DNA was unsurpsingly, not in a perfect condition. For this study it was split up into 250 fragments. Each one was sequenced many times to ensure the accuracy of the results.
Mitochondrial DNA changes very slowly over time and so provides a good medium for comparison with modern day genomes. In this case Oetzi's ancient DNA was compared with the genomes of a group of people that comprise a haplogroup. A haplogroup is simply a collection of people who share common ancestral genetic sequences. Oetzi's DNA was compared with the K1 haplogroup, and his lineage was completely different to all others.
From this, the scientists led by Professor Franco Rollo at the University of Camerino concluded that either Oetzi's lineage is extremely rare or it has died out. Either way, at the moment it's not seen in any modern day humans.
This new research overturns a previous genetic study of Oetzi's mitochondrial DNA that suggested he may have modern day relatives living in Europe.