The concept of transition mutation is very vital to genetic biology. In the field of genetics, the transition point is the one that changes the purine nucleotide into another purine nucleotide (or a pyrimidine nucleotide into another pyrimidine nucleotide). There are different reasons why this transitions might be caused.
The most commonly known reason is that transitions are caused due to the oxidative deamination. Having made that clear, the chemical 5-Methylcytosine is far more likely to make a transition in comparison to unmethylated cytosine. This is probably due to a deamination that is spontaneous and occurs without a definite pattern/time.