The 1.2.0 release, "Enoch Root", contains an important fix to SubUint64, which was flaweed (see https://github.com/holiman/uint256/pull/82 ). Other than that some API methods have been changed, to make uint256.Int closer resemble big.Int, to be easier to use as a drop-in replacement (also https://github.com/holiman/uint256/pull/82 ).