circRNA is a new type of non-coding RNA (ncRNA) produced by non-sequential exon, intron, or both backsplicing. Their covalently closed loop feature, which implies they lack 5' end caps and 3' poly-A tails, differentiates them. Because of their nuclease resistance, circRNAs are highly stable forms of ncRNAs. When circRNAs were unearthed in the 1970s, they were mistakenly believed to be viral genomes or byproducts of mis-splicing events. However, recent research has revealed that circRNAs have important biological functions and are evolutionarily conserved across plants, animals, and humans.