This came up recently: Romans 5:13 is in parenthesis in the King James version. Could you please explain why?
The parenthesis was added by the translators, both in the King James and New King James versions, to emphasize that this is explaining the charge made in Romans 5:12. Paul stated, "Therefore, just as through one man sin entered the world, and death through sin, and thus death spread to all men, because all sinned." Since sin is the breaking of law (I John 3:4), it would be natural to wonder how people could be justly charged with sin before the law of Moses came into being thousands of years later. The answer is: "For until the law sin was in the world, but sin is not imputed when there is no law. Nevertheless death reigned from Adam to Moses, even over those who had not sinned according to the likeness of the transgression of Adam, who is a type of Him who was to come" (Romans 5:13-14). You can't have sin without law; thus Paul is arguing that law existed from Adam to Moses. It was more than one law because everyone after Adam did not commit the same sin that Adam committed for the simple reason that the Tree of the Knowledge of Good and Evil wasn't available to them. Therefore, there was a set of laws in place in the time between Adam and Moses (Romans 2:14-15). Some you can see in the covenant made with Noah in Genesis 9, which affirms that murder is sinful. You can see it when Pharaoh becomes upset with Abram for almost causing him to commit adultery unwittingly (Genesis 12:18). Law existed; therefore, sin was present.
Unfortunately, many commentators miss this important argument. They are so certain that there was no law between Adam and Moses that they assume that Paul is saying that no one in that time frame was charged with sin, other than the inherited sin of Adam which they firmly believe is passed on to all mankind. But it leaves them a huge flaw. If mankind between Adam and Moses were equally sinful, then how did mankind get so sinful before the flood that God decided He had to wipe out man? If there was no law, then there could not have been that much sin. No, Genesis 6 demonstrates to us that there were laws which people were violating continually.
The translators of the King James and New King James saw these verses as a side comment in the main thread of Paul's arguments, so they framed them in parenthesis. Most other translations don't bother.