Under this scenario, adaptive walks can be modeled as a simple Markov chain on the space of possible fitness ranks with an absorbing state at i= 1, from which no beneficial mutations are accessible.
Mutations are accessible in the Human Mutation Database and Leiden Open Variation Database.
This supports the intuitive understanding that if more beneficial mutations are accessible, the population size that optimizes the trade-off between the speed of adaptation and the magnitude of the adaptive response shifts towards larger populations.
Our findings show that KIT and BRAF mutations, which are accessible for present targeted therapies, are only rarely present in SNMMs, whereas NRAS mutations seem to be relatively more frequent.
However, a mutation favored by selective forces will fix more frequently than one just driven just by stochastic forces, so the virus often fixes immune-escape mutations once they are accessible, explaining why the destabilizing mutations are enriched in CTL epitopes.
In the case when patients carrying certain genetic mutations are not accessible, wild-type stem cells can also be introduced with genetic variants to create "patient" cell lines (Fig. 1A).
The Gly46Asp and Phe52Leu mutations are solvent accessible and occur in β-strand and turn regions respectively within the N-terminal region (and distal from the catalytic sector).
However, such mutations are not accessible to E. coli due to the presence of multiple copies of the 16S rRNA gene, a property common to many fast-growing bacteria.
Allowing only single forward mutations, there are 5! = 120 possible pathways, of which 18 were found to be accessible by beneficial mutations alone [1].
An additional argument for calling all these mutations compensatory is that they can produce a fitness gain only in association with one or more mutations that would otherwise be deleterious, that is, none of the mutations that could be termed advantageous is accessible directly from the main neutral network.
