The F13L (p37) are the most abundant of the poxviral membrane proteins . Coding for 37 KDalton (372 amino acids) , the F13L contains two residues that can be palmitoylated ( 185 CC), two phospholipase-like motifs (~ 121 xxxxDD and near-canonical 312 NxKxxxxD) , and one motif to interact with cellular TIP47 ( 253 YW) . In vaccinia, F13L co-localizes with B5R (B6R orthologous in MPXV) , which could be also palmitoylated ( 301 C 303 C) . B5R(B6R) belong to transmembrane anchored glycoproteins of 42 KDalton (317 amino acids) containing an extracellular domain, a transmembrane α-helix and a short cytoplasmic tail . F13L and B5R proteins are both required for vaccinia poxviruses to be released from their infected host-cells 17 F13L and/or B5R deletions cause inhibition of membrane wrapping, fewer extracellular viral particles and attenuation of the resulting poxviruses .