Factors which affect plasmid copy number include the host strain, the insert, incubation time, the culture inoculum, growth conditions, and temperature and extracellular stresses. While most E. coli strains can propagate plasmids, those lacking the endonuclease EndA (endA-) are optimal for high plasmid yields. Plasmids carrying large inserts or genes producing toxic products tend to be maintained in fewer copies by bacteria. Longer incubation periods (12-16 hours) often result in higher plasmid copy numbers. Additionally, freshly streaked bacteria and freshly transformed cells generally yield higher plasmid copy numbers. Lastly, aeration levels, culture volume, antibiotic presence, temperature, and growth medium all impact copy number. Some origins of replication (ori) are temperature-sensitive, while others can be induced to amplify more copies with specific additives like chloramphenicol.