WYL_2 is a family of Sm-like SH3 beta-barrel fold containing domains. WYL is named for three conserved amino acids found in a subset of domains of this superfamily. These residues are not strongly conserved throughout the family. Rather, the conservation pattern includes four basic residues and a position often occupied by a cysteine [1], which are predicted to line a ligand-binding groove typical of the Sm-like SH3 beta-barrels. It is predicted to be a ligand-sensing domain that could bind negatively charged ligands, such as nucleotides or nucleic acid fragments, to regulate CRISPR-Cas and other defense systems such as the abortive infection AbiG system [1]

1: CARF and WYL domains: ligand-binding regulators of prokaryotic defense systems. Makarova KS, Anantharaman V, Grishin NV, Koonin EV, Aravind L; Front Genet. 2014;5:102. PMID:24817877