Boss Stem Cells

All four Boss Source Cells are acquired by killing The Hand Of The King. Once you get the first, you need to beat him with one cell inoculated to get the second, with two cells inoculated to get the third, etc. Boss Source Cells can be inoculated at the start of the game in the Tube, which applies difficulty modifiers. When inoculated, they also give you access to bonus doors scattered in the levels. The doors show the number of cells required and shine blue if you can enter them.