Cover image

Cover L1 retrotransposon is an abundant inhabitant of mammalian genomes and an important force in mammalian genome evolution. In addition to its role as an insertional mutagen and a potent substrate for homologous recombination, L1 has produced a number of new genes by reverse transcribing cellular mRNAs and integrating the resultant cDNAs back into the genome. An unusual product of such L1-mediated retrotransposition process is a new primate gene PIPSL, which originated from a read-through, chimeric transcript between the neighboring phosphatidylinositol-4-phosphate 5-kinase (PIP5K1A) and the 26S proteasome subunit (PSMD4) genes. The PIPSL gene was formed roughly 17 million years ago, before the divergence of humans and orangutans, and was transcribed and translated to form a functional protein in an ancestral primate. The image background shows an outdoor sculpture of two gorillas, in the foreground, Sugi, a playful 11-year-old orangutan, both in the Philadelphia Zoo. (Cover illustration and photos by D. Babushok and H. Kazazian. [For details, see Babushok et al., pp. 1129–1138.])