FIGURE 1.

Brief discovery history of eccDNA. The technologies and key discoveries were illustrated following the timeline of the milestone studies. The detailed descriptions are as following: in 1965, Alix Bassel and Yasuo Hoota first observed eccDNAs in boar sperm under the electron microscope (Hotta and Bassel, 1965). The Roger’s group extracted eccDNA in HeLa cells by CsCl gradient purification in 1976 (Radloff et al., 1967). In 1984, Krolewski found that eccDNA molecules were homologous to genomic DNA (Krolewski et al., 1984) using Southern blot indicating that eccDNAs may be derived from chromosomes. Neutral–neutral two-dimensional (2D) gel electrophoresis combined with standard procedures of Southern blots and hydridization verified useful for characterizing eccDNA including its size range and sequence content, and it can prove eccDNA organized as discrete multimers (Brewer and Fangman, 1987; Cohen and Mechali, 2001). With rapid advances in high-throughput DNA sequencing, more eccDNAs have been identified and quantified on a global level (Turner et al., 2017). Furthermore, the architecture of eccDNA can be directly observed by super resolution (SR) confocal microscopy (Sihan Wu, 2020).