Not a race or clade, so much as a dispersed ideological movement, similar in this regard to the Mortalists (In fact, there is significant overlap here, though not all Mortalists are Kassians, all Kassians are in effect Mortalists).
Broadly speaking, Kassians (who comprise everything from unmodified baseline Hu, to Hu-friendly SI:1 AIs) maintain that genemods (except to correct invariably terminal metabolic disorders, such as Tay Sachs, Cystic Fibrosis, Hemophilia, etc) should be eschewed completely, by Humans and other "naturally-evolved/adapted" species (This definitely - especially - includes genemods which confer extended lifespans or effective immortality, which is why Kassians are all effectively Mortalists even if ey don't specifically identify or think of emselves as such), and that production of new sophont bionts should only be through means initially afforded by Nature.
Far from being strictly ideological, Kassians maintain that there is logic to eir view. Kassians point-out that Homo Sapiens initially came about through numerous random mutations which, by themselves, seemed to confer no advantage and were often plainly detrimental at first (weak jaws, and heads almost too big for the Human female birth canal, being among them), and that Humans ultimately became what ey did (sapient, intelligent, tool-making social beings, and all) through adapting to these "disadvantages" through post-natal, non-genetic means (becoming better tool-makers and tool-users, and sticking together in larger social groups with more complex internal dynamics to provide support and care for new mothers and offspring who would have died otherwise). Kassians definitely emphasize the role of "nurture" over "nature" in this regard.
Kassians point-out that natural evolution, which relies on unplanned mutations to create variety (initially producing variations completely without regard for whether they will be "useful" or "beneficial"), is less inherently limited than "intelligently designing" new life forms, because it's not limited by what it can "think" of, or by what it considers beneficial or useful. Thus, ironically, natural evolution driven by random mutations and guided by the "invisible hand" of natural selection, may inadvertently come up with many useful or beneficial things that an "intelligent" designer may never think of, either because they never crossed eir mind or e thought they would only be harmful or at best useless.
"Sometimes mindless processes, lacking any foresight or planning ability, or any notion beforehand of what would be 'good' or 'bad', can be far more ingenious and creative than the most brilliant mind. If you limit yourself only to what you can think of, or what you think beforehand will be beneficial or useful, you are really limiting yourself!" - Kassian slogan.