The Friend spleen focus-forming provirus is 6,296 base pairs (bp) in length. Compared to Moloney murine leukemia virus, it has undergone five major deletions, three substitutions, and a number of minor alterations. Otherwise, these viruses are about 90% homologous. A 16-bp palindrome is found in the region thought to be involved in packaging and dimerization of the RNA genome. Premature termination of translation of the gag polyprotein is attributed to a 13-bp deletion in the p12 region. A substitution of xenotropic env sequences was identified in the 5' region of the env gene; 150 nucleotides 3' to this substitution, a deletion of 585 bp removes the site where the normal env precursor protein is cleaved to form gp70 and p15(E), resulting in a fusion protein of Mr 44,725. Due to these changes, the env product gp55 is expected to have a substantially different conformation on the cell surface compared to either a xenotropic or ecotropic gp70 protein, and may be responsible for the rapid erythroleukemic potential of spleen focus-forming virus.