Wesker's Notes on Differing Mutations is a file that can be found in Resident Evil: The Umbrella Chronicles.
I have a theory on Sergei's unique course of evolution.
I had always assumed that the variety of effects the virus brings about on its host were mere random mutations.
However, witnessing first-hand the effects on Sergei combined with what I know of Marcus's transformation points to a common thread between their outcomes.
The host's mindset appears to influence the evolution of the virus.
Sergei mutated into a monster whose very body is the picture of suffering as he was ripped apart from the inside by thorns. I can only imagine that his transformation represents the true mindset of the person he was inside.
While I have no proof, my intuition tells me that this must be the case. If the personality of the host can truly affect the course of the virus manifestation, where does that leave me?
Sergei's evolution led me to a hypothesis.
I firmly believed that the direction of evolution in virus carriers was determined purely by chance.
But when I saw that man's transformation, a certain reasoning came to my mind that I'd also felt when I heard of Marcus turning into a monster.
I wonder if the will of the carrier has a strong effect on evolution.
That form of Sergei was a manifestation of his suffering, admonished and torn by his inner thorns.
It was nothing more than the embodiment of that man's innermost thoughts.
There's no evidence. Only my hunch tells me so.
One's own thoughts determining the direction of virus evolution...
If this is true, I wonder how this would transform me.
Wesker's exact description of what he came to from witnessing Vladimir's transformation was different between the two versions: In the Japanese version, he states he was led to a hypothesis, while the English version mentions that he has a theory. In addition, the Japanese version also has Wesker indicating that Sergei Vladimir had some inner pain that was influencing the transformation, which is never directly indicated in the localization but nonetheless implied. Wesker also directly ponders how the virus would transform him if his hypothesis/theory is true, something that's implied but not directly stated in the localization.
Although Wesker's memo implies that the prototype virus would result in him mutating by will, this was never demonstrated in his later appearances.