Replication is enhanced by infection of mitotically active cultures. Truncation of chromosome 21 resulted in a human artificial chromosome that was mitotically stable. However, its mechanism may involve mitotically stable genetic memory. There are three benign variants&colon; bizarre (atypical); cellular; and mitotically active. The Oogonia multiply by dividing mitotically; this proliferation ends when the oogonia enter meiosis. The resulting HAC was mitotically stable, and correctly expressed dystrophin in chimeric mice. The basal layer of the epithelium is the most mitotically active and reproduces new cells. Epigenetic mutations such as DNA methylation are mitotically heritable, but also reversible, unlike gene mutations. Each microspore then divides mitotically to form an immature microgametophyte called a pollen grain. These cells divide mitotically to form either larger, multicellular individuals, or more haploid cells. These gametophytes, after reaching the appropriate substrata, grow mitotically to eventually produce gametes. "Type Ap spermatogonia" repeatedly divide mitotically to produce identical cell clones linked by cytoplasmic bridges.