Another problem arose from the presence of NUMT in the genome associated with the hardship of concluding the exact number of mitochondrial insertions into the nDNA. Determining the exact number of NUMT pseudogenes for a species is difficult task for several reasons. One reason that makes detection of NUMT sequences more difficult is the alteration of these sequences by mutation and deletion. Two further substantial obstacles make recognition of NUMT very difficult; first is the lack of correlation between the proportion of noncoding nDNA and the number of NUMT inserts in the nuclear genome. That is, NUMT insertion could occur in the known or predicted coding region, both intron and exon, rather than only in intergenic and intronic region. Second, mitochondrial DNA integrated into animal nuclear genomes is primarily limited to animals with circular mitochondrial genomes without introns. NUMT studies are not available in animals with linear mitochondrial genomes or those with intron-containing mitochondria. Therefore, despite all the available advanced technologies, it remains to be determined whether NUMT transposition differences exist between circular and linear mtDNAs.