I like this as a example just how much the imperium cares about xeno races even those living under an Imperial protectorate during the great crusade.
The Adarnian were a Xenos species that were discovered by the Imperium, during the Great Crusade
However they were decreed to be harmless and were allowed to live under an Imperial protectorate[. Unfortunately for the Adarnian, though, their genetic makeup was soon discovered to have miraculous restorative effects on Humans, when their bodies were rendered down to a fluid. This elixir, which was injected into the bloodstream, became the last resort for Humans, when all other Rejuvenats had failed and the Adarnian were soon harvested to extinction
Also the last transmission from another xeno race The Diasporex before being wiped out.
"We only wished to be left alone"