"In the summer of 1944 at Cold Spring Harbor Laboratory, McClintock systematic studies on the mechanisms of the mosaic color patterns of maize seed and the unstable inheritance of this mosaicism.^[44] She identified two new dominant and interacting genetic loci that she named Dissociation (Ds) and Activator (Ac). She found that the Dissociation did not just dissociate or cause the chromosome to break, it also had a variety of effects on neighboring genes when the Activator was also present, which included making certain stable mutations unstable. In early 1948, she made the surprising discovery that both Dissociation and Activator could transpose, or change position, on the chromosome.^{[45][46][47][48]"}

"Between 1948 and 1950, she [McClintock] developed a theory by which these mobile elements regulated the genes by inhibiting or modulating their action. She referred to Dissociation and Activator as "controlling units"—later, as "controlling elements"—to distinguish them from genes. She hypothesized that gene regulation could explain how complex multicellular organisms made of cells with identical genomes have cells of different function.^[52] McClintock's discovery challenged the concept of the genome as a static set of instructions passed between generations.^[3] In 1950, she reported her work on Ac/Ds and her ideas about gene regulation in a paper entitled "The origin and behavior of mutable loci in maize" published in the journal Proceedings of the National Academy of Sciences. (Wikipedia article on Barbara McClintock, accessed 3-2020). Digital facsimile from pnas.org at this link.

In 1983 McClintock was awarded the Nobel Prize in Physiology or Medicine "for her discovery of mobile genetic elements." See also Nos. 13560 and14072.

McClintock (Nobel Prize 1983) discovered transposable elements or jumping genes. She found that certain parts of chromosome had switched position. This refuted the then-popular theory that genes were fixed in their position on a chromosome. McClintock found that genes could not only move but they could also be turned on or off due to certain environmental conditions or during different stages of cell development. Digital facsimile from PubMedCentral at this link.

In this paper McClintock described a novel mobile genetic element that she called Suppressor-Mutator (Spm), and its complex regulation. She discovered that Spm could switch back and forth between an “inactive” form and an active form—what she called “changes of phase,” later understood to be a result of methylation. Some forms of Spm cycled between inactive and active phases during development, while others showed specific patterns of expression, and were only active in certain plant parts. She suggested that "there was a direct relation between the degree of supressive capacity of an Spm element and its ability to induce a mutation." These pioneering studies foreshadowed later work by others showing the importance of epigenetics— heritable changes in development not caused by changes to the DNA sequence. Digital facsimile from the Internet Archive at this link.

(Thanks to Juan Weiss for this reference and its interpretation.)