Nuclease that is responsible for viral evasion of host cGAS-STING innate immunity. Cleaves 2',3'-cGAMP which is produced by host cGAS following recognition of cytosolic DNA and blocks the subsequent 2',3'-cGAMP-mediated activation of TMEM173/STING, which normally spreads to adjacent cells and activates the interferon and NF-kappa-B immune responses. 2',3'-cGAMP + H2O = Gp(2'-5')Ap(3') + H(+) Homodimer. The substrate binding site is formed by the N-terminus of a monomer and the C-terminus of the opposite monomer. In some poxviruses, the poxin is expressed as a poxin-schlafen fusion protein. In the N-terminal section; belongs to the poxin family. In the C-terminal section; belongs to the Schlafen protein family. Subgroup poxviridae B3 subfamily.