design: Circular extrachromosomal DNA (ecDNA) was enriched by Plasmid Safe DNase treatment to remove linear DNA. Purified circular DNA was amplified by rolling circle amplification (RCA) using Phi29 polymerase, debranched with T7 endonuclease I, and size-selected. Nanopore ligation libraries were prepared using the ONT Ligation Sequencing Kit and sequenced on a GridION using R10.4.1 flow cells.