why snp class classified into 6 class?

Question

[9521ljh]

Using maftools, plotmafSummary command.

the plot only show 6 class SNP( T>G, T>A, T>C, C>T, C>A, C>G).

why it is not show G > A or A>G , etcs....?

[cpad0112]

copy/pasted: Because the pyrimidine base of the Watson-Crick base-pair has become a community standard.

https://github.com/PoisonAlien/maftools/issues/510

in the code (https://rdrr.io/bioc/maftools/src/R/TrinucleotideMatrix.R):

 #substitutions are referred to by the pyrimidine of the mutated Watson-Crick base pair