First, SIRV2 aca8 (gp01/gp54) and both known Acr genes, acrID1 (gp03) and acrIIIB1 (gp48) are expressed immediately post infection of Sulfolobus islandicus LAL14/1 (Supplementary Fig. 2A)35; second, most of the small hypothetical genes at the SIRV2 genomic termini, including acrID1 and acrIIIB1, share a highly similar regulatory sequence, in a sharp contrast to the very low sequence conservation of the nucleotide sequences of the genes themselves and limited similarity among the amino acid sequences of the encoded proteins (Fig. 1B). Here, ACR is linked to infection.