The name of this sequence is “Alter Egos”. This is story related and cannot be missed. Just complete each of the following memories highlighted by the icon on the map.
All Things Come To He Who Waits
Note: In order to advance to DNA Sequence 14, you have had to collect all 30 Codex pages scattered throughout the four cities (there are none in Monteriggioni). If you haven’t obtained them all, Mario will send you back out to collect them. Once you complete that task, they must be deciphered by Leonardo before they can be placed on the Codex Wall.