- A: a+b three layers
- X: A subdomain in the anticodon-binding domain of a subclass of class I aminoacyl-tRNA synthetases
- H: A subdomain in the anticodon-binding domain of a subclass of class I aminoacyl-tRNA synthetases
- T: A subdomain in the anticodon-binding domain of a subclass of class I aminoacyl-tRNA synthetases
- F: