Inactivation of UL17 gene is lethal for MDV replication. (A) Schematic representation of the genomic localization of the MDV UL17 gene. The unique long (UL), unique short (US), and repeated (IR, TR) sequences are indicated. (B) Southern blot analysis of HindIII digests of pRB-1B (lane 1) and pRB-1B∆17 (lane 2). Hybridization with a KanR specific-probe reveals the insertion of the cassette in the UL17 locus. (C) Sequence analysis of UL17 gene confirming the alteration of codon 21 from ATG (CAT in the complementary strand) in pRB-1B (WT) to TAG in the pRB-1B17stop mutant (MUT). (D) vRB-1B17Stop mutant viruses does not spread to neighbouring cells: CESC transfected with pRB-1B17stop DNA were fixed at 120 h p.i. Co-staining of VP22 (green) and VP5 (red) shows replication of mutant virus limited to the transfected cell.