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.)