Tests of sperm DNA damage and sperm chromosome anomalies have also been developed. The advanced sperm tests yield information on the fertilizing capacity of human spermatozoa, as well as their ability to support normal embryonic development. The most common abnormalities involve the structure of the sperm tail, with defects in either the axoneme or outer dense fibers. Postcoital test (PCT) assesses the ability of sperm to traverse the cervical mucus, and is performed by examining the cervical mucus several hours after intercourse for the presence of sperm. The sperm penetration assay (SPA) utilizes hamster ova, which have had the zona pellucida enzymatically removed, allowing penetration by human spermatozoa. The hemizona assay uses zona pellucida from nonfertilizable, nonliving oocytes. Reactive oxygen species (ROS) are ubiquitous in aerobic biologic systems and are formed as a by-product of oxygen metabolism.