The Cell-Kin are a minor Xenos species of the Technetium Belt that came into conflict against the Subjugators Chapter in a protracted campaign. The unique reproductive cycle of the species made their complete eradication very difficult to achieve. The Cell-Kin reproduce by viral dissemination, their DNA infecting the body of another creature where it would literally reshape the host in its own image.
The Space Marines' enhanced genetic make-up proved largely resilient to this thread, but twenty of the brethren of the third company were lost in the early stages of the campaign, they were slowly mutated into new, hideous forms, leading to permanent metamorphosis, ramdom mutations or more subtle but equally damaging changes. The Chapter's Apothecaries identified the threat, but they were to late to save the Battle-Brothers affected by the Cell-Kin's infection. The infected brethren that survived the conflict escaped, and are assumed to be at large in the galaxy to this day.