Anomalous base pairing of nucleotide tautomers
The tautomers are rare but they can form and it is suggested that tautomerization can lead to mutations because of non-cognate base pairing. 

Khuu &amp; Ho (2009), have inferred the presence of adenine and thymine tautomers from the crystal structure of an in-vitro assembled holliday junction. They infer that imino-Adenine base pairs with amino-cytosine (normal form) and enol-thymine base pairs with keto-guanine (normal form). 

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;


  Reproduced from: Khuu &amp; Ho (2009)




Khuu, P., &amp; Ho, P. S. (2009). A rare nucleotide base tautomer in the structure of an asymmetric DNA junction. Biochemistry, 48(33), 7824-7832.