Researchers showed that mutations caused by either a single base insertion (+) or a single base deletion (-) could be "suppressed" or restored by a second mutation of the opposite sign, as long as the two mutations occurred in the same vicinity of the gene. This led to the conclusion that genes needed to be read in a specific "reading frame" and a single base insertion or deletion would shift the reading frame (frameshift mutation) in such a way that the remaining DNA would code for a different polypeptide than the one intended. Therefore, researchers concluded that the second mutation of opposite sign suppresses the original mutation by restoring the reading frame, as long as the portion between the two mutations is not critical for protein function.