This is because during viral replication, the nucleic acid and viral proteins are not always produced in 1:1 ratio and viral assembly process results in complete virions as well as empty capsids and/or excess free viral genomes. In the example of foot-and-mouth disease virus, the ratio of whole virions to RNA copies within an actively replicating host cell is approximately 1:1000.[27] Products for qPCR-based virus titration are available commercially through numerous companies (e.g. Invitrogen, Roche or Qiagen). Advantages of titration by qPCR include quick turnaround time (1–4 hours) and sensitivity (can detect much lower concentration of viruses than other methods).