DNA topoisomerase I and II are nuclear enzymes, and type II consists of two highly homologous isoforms: topoisomerase IIa and IIb. These enzymes regulate the topology of DNA, maintain genomic integrity, and are essential for processes such as DNA replication, recombination, transcription, and chromosome segregation by allowing DNA strands to pass through each other. Topoisomerase I nicks and rejoins one strand of the duplex DNA, while topoisomerase II transiently breaks and closes double-stranded DNA. Topoisomerases are very susceptible to various stresses. Acidic pH or oxidative stress can convert topoisomerases to DNA-breaking nucleases, causing genomic instability and cell death. DNA-damaging topoisomerase targeting drugs (e.g., etoposide) also convert topoisomerases to nucleases, and the enzyme is usually trapped as an intermediate, covalently bound to the 5+ end of the cleaved DNA strand(s). Research studies have shown that this intermediate leads to genomic instability and cell death, and thus, agents that target topoisomerases are highly sought after cancer chemotherapeutic drugs. Ca2+-regulated phosphorylation of topoisomerase IIa at Ser1106 modulates the activity of this enzyme and its sensitivity to targeting drugs.