# Constraints

Many puzzles place restrictions on player solutions. Constraints increase difficulty in puzzles, or help players avoid sequences that can cause synthesis of lab submissions to fail. In player puzzles not designed as lab candidates, players can require a minimum number of AU or GU pairs. The required number of GU pairs cannot exceed 33%. Players can also set a maximum on the number of GC pairs. Puzzles designed as lab candidates may not have more than 3 consecutive guanine or cytosine nucleotides. Some challenge puzzles may also limit the number of consecutive G or C nucleotides in other ways.

## Keeping Track of Restricted Sequences

Reminders of constraints are shown at the top left of the screen. The border of a restriction box turns red if it is not satisfied. The numbers in the boxes display information concerning the constraint. In puzzles that limit the number of consecutive nucleotides, enabling the option "highlight restricted sequences" can be helpful.

- GU Minimum: Displays the number of GU pairs in the structure and the minimum required.
- AU Minimum: Displays the number of AU pairs in the structure and the minimum required.
- GC Maximum: Displays the number of GC pairs in the structure and the maximum allowed.
- Consecutive G or C Nucleotide Limit: Displays the highest number of consecutive G or C nucleotides and the maximum number allowed.

## (Possible) Future Restricted Sequences

Players have requested capability for constraints that may be available in Eterna in the future. These would follow International Union of Pure and Applied Chemistry IUPAC notation standards for matching more than one value of a base.