Polymers (i.e., molecules composed of repeating atomic units within the molecule) that repeatedly branch (while “growing” due to addition of more atoms in a repeating pattern) until that branching is stopped by the physical constraint of contacting itself (i.e., having formed a complete, hollow sphere). Discovered during the 1970s by Donald Tomalia, dendrimers possess sites on their exterior surface to which genetic material (e.g., genes or other portions of DNA) can be “attached.” Dendrimers bearing such genetic material have been shown to be able to successfully transfer that genetic material into more than thirty types of living animal cells.