In addition to chromosomal DNA (chr-DNA) and mitochondrial DNA, eukaryotic cells contain extrachromosomal DNA (ec-DNA). Analysed extrachromosomal circular DNA (ecc-DNA) accounts for up to 20% of the total cellular DNA. Ecc-DNAs contain coding and non-coding sequences originating from chr-DNA and mobile genetic elements (MGEs). MGEs include sequences such as transposons, which have the potential to move between different and the same DNA molecules, thereby, for example, causing rearrangements and inactivation of genes. Ecc DNAs have aroused interest in diseases such as malignancies and diagnostic procedures relating to this. A database to collect ecc-DNA has been established. Investigations are needed to find possible differences in sequences of chr-DNA after sequencing the whole cellular DNA (WCD), namely: chr-DNA plus ec-/ecc-DNA compared to chr-DNA, which is separated from ec-/ecc-DNA. Standards for sequencing protocols of WCD have to be developed that also reveal the sequences of ecc-DNA; this concerns “single-cell genomics” in particular.