Kiessling is noted for her discovery of reverse transcriptase activity in normal human cells. This report pioneered the importance of naturally occurring retrovirus sequences in human genes, now thought to be important to the genetic plasticity involved in human evolution and biology. Prior to this discovery, it had been assumed that reverse transcriptase was an enzyme found only in retroviruses (such as human immunodeficiency virus). To understand the normal biologic role of reverse transcriptase, Kiessling began to study eggs and early cleaving embryos. Her dual interests in virology and reproductive biology led to research in semen transmission of human immunodeficiency virus, and the creation of the first laboratory for human in vitro fertilization in Oregon in the early 1980s. Harvard Medical School recruited Kiessling in 1985, where she conducted research until 2011. Kiessling currently conducts research at the Bedford Stem Cell Research Foundation.