The 'Malformations of G' expansion contains two highly detailed models for the extra mutations of Dr William Birkin, along with rules and behavior cards for each of these boss models. Also included are updated scenarios and tiles for the core game, allowing players to encounter these deadly enemies as part of their games
—description (mostly)from the publisher
Although originally planned to have 4 of Birkins' forms, It was later split in to 2 boxes. This box contains Birkin stage 2 & 4 which appear in the core or A-File story