In general, mutations can happen anywhere on any gene, so every patient’s cancer will have its unique signature of mutations. However, like in the evolution of organisms by natural selection, most random mutations will have a detrimental effect and the cells carrying it will die. Some of the mutations will be neutral and despite the change in the amino acid, the cells harbouring it won’t survive better or worse than cells that don’t have it. But a few mutations will make the cancer cells proliferate faster or evade the immune system better, which will lead to these cells surviving and ultimately overtaking the population of the cancer cells. The latter mutations often happen in the same places on the same genes, and in melanoma for example, in as many as 41% of cases the 600th amino acid in a protein called BRAF mutates from valine to alanine (so the code for that mutation is “BRAF V600E”), and BRAF is only one example of such genes that commonly mutate in the same position.

So to answer your question - I don’t know Moderna’s exact protocol, but my guess is that the tailored vaccine will contain a mixture of these commonly occurring mutations and some mutations that are unique to the patient.