In the summer of 1944 at Cold Spring Harbor Laboratory, McClintock began systematic studies on the mechanisms of the mosaic color patterns of maize seed and the unstable inheritance of this mosaicism. 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.She observed the effects of the transposition of Ac and Ds by the changing patterns of coloration in maize kernels over generations of controlled crosses, and described the relationship between the two loci through intricate microscopic analysis. She concluded that Ac controls the transposition of the Ds from chromosome 9, and that the movement of Ds is accompanied by the breakage of the chromosome. When Ds moves, the aleurone-color gene is released from the suppressing effect of the Ds and transformed into the active form, which initiates the pigment synthesis in cells. The transposition of Ds in different cells is random, it may move in some but not others, which causes color mosaicism. The size of the colored spot on the seed is determined by stage of the seed development during dissociation. McClintock also found that the transposition of Ds is determined by the number of Ac copies in the cell.Between 1948 and 1950, she 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. McClintock's discovery challenged the concept of the genome as a static set of instructions passed between generations. 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. In summer 1951, she reported her work on the origin and behavior of mutable loci in maize at the annual symposium at Cold Spring Harbor Laboratory, presenting a paper of the same name. The paper delved into the instability caused by Ds and Ac or just Ac in four genes, along with the tendency of those genes to unpredictably revert to the wild phenotype. She also identified "families" of transposons, which did not interact with one another.Her work on controlling elements and gene regulation was conceptually difficult and was not immediately understood or accepted by her contemporaries; she described the reception of her research as "puzzlement, even hostility". Nevertheless, McClintock continued to develop her ideas on controlling elements. She published a paper in Genetics in 1953, where she presented all her statistical data, and undertook lecture tours to universities throughout the 1950s to speak about her work. She continued to investigate the problem and identified a new element that she called Suppressor-mutator (Spm), which, although similar to Ac/Ds, acts in a more complex manner. Like Ac/Ds, some versions could transpose on their own and some could not; unlike Ac/Ds, when present, it fully suppressed the expression of mutant genes when they normally would not be entirely suppressed. Based on the reactions of other scientists to her work, McClintock felt she risked alienating the scientific mainstream, and from 1953 stopped publishing accounts of her research on controlling elements.

Question: What does the acronym SPM stand for?
Answer: Suppressor-mutator

Question: What were controlling units later known as?
Answer: controlling elements