i, alone

Yet some organisms do possess genes, but moreso for detoxification and sequestration of lead, humans included.

However, polymorphic forms of preexisting genes could potentially affect: polymorphic forms of the delta-aminolevulinic acid dehydratase gene (ALAD), which is involved in the heme biosynthesis pathway, has actually been shown to affect blood and bone lead levels.

So, if say, increased expression of both polymorphic forms of the gene, or a gain of function mutation that allows for constitutive expression via pluripotency and morphogenic redifferentiation (somehow).

There's also the vitamin D receptor VDR gene, which polymorphic forms could also influence accumulation of lead in bone.

This could also potentially explain why the Engine's effects didn't work on everyone: Polymorphisms are variants of alleles (alternate versions of genes) that only some proportion of the population posseses.

Unsure % of population that possess these polymorphisms, though, since you do see massive phenotypical changes in most Variants' cases.

It could be that the nanofactory (expression of lead tumors) part did work (for some) but the control part didn't (but that's kind of out of my realm of understanding).

But that would also mean heavy external lead influence (ie, Murkoff treating victims with lead).

tl;dr. Amendment to my "it's most likely iron based tumors" comment on my last post— it possibly could be lead, but still unlikely as we don't actually use lead in biological processes, unlike iron.

Then you remember MKUltra CIA's Hypnotic Homicide file (That Miles found) — clearly, they wanted whole organism level control, through hypnosis. By influencing hypnosis on beings capable of understanding direction/action, they would not have to start from scratch, allowing them a human-based weapon at their disposal.

Additionally embryos would possibly not have enough cell mass to host the Walrider (not enough nanofactories to sustain homeostasis and repair).

And possibly because, at embryonic level (if assuming they work with humans), the project would take much longer awaiting development of said embryo, which may not even get very far if they differentiated most of it into nanofactories, as they redirected wildtype (conventional) morphogenic development into nanofactories (which presumed does not retain regular developmental processes), which brings me back to the first point: they want to have hypnotic control. Like an input/control board. You can't do that with a lump of cancer cells.)

Morphogens don't really have an effect on fully differentiated somatic cells, generally.