The nucleotide sequence of the 3913 base pair XhoI O fragment located in an evolutionary variable region adjacent to the right end of the genome of ectromelia virus (EMV) was determined. The sequence contains two long open reading frames coding for putative proteins of 559 amino acid residues (p65) and 344 amino acid residues (p39). Amino acid database searches showed that p39 is closely related to vaccinia virus (VV), strain WR, B22R gene product (C12L gene product of strain Copenhagen), which belongs to the family of serine protease inhibitors (serpins). Despite the overall high conservation, differences were observed in the sequences of p39, B22R, and C12L in the site known to interact with proteases in other serpins, suggesting that the serpins of EMV and two strains of VV may all inhibit proteases with different specificities. The gene coding for the ortholog of p65 is lacking in the Copenhagen strain of vaccinia virus; the WR strain contains a truncated variant of this gene (B21R) potentially coding for a small protein (p16) corresponding to the C-terminal region of p65. p65 is a new member of the family of poxvirus proteins including vaccinia virus proteins A55R, C2L and F3L, and a group of related proteins of leporipoxviruses, Shope fibroma and myxoma viruses (T6, T8, T9, M9). These proteins are homologous to the Drosophila protein Kelch involved in egg development. Both Kelch protein and the related poxvirus proteins contain two distinct domains. The N-terminal domain is related to the similarly located domains of transcription factors Ttk, Br-C (Drosophila), and KUP (human), and GCL protein involved in early development in Drosophila. The C-terminal domain consists of an array of four to five imperfect repeats and is related to human placental protein MIPP. Phylogenetic analysis of the family of poxvirus proteins showed that their genes have undergone a complex succession of duplications, and complete or partial deletions.