Computer search for homology regions of retroelement ORF1 protein products and retroviral gag-encoded proteins was performed using nucleotide and protein sequences of 113 retrotransposons, conservative domains were found in amino acid sequences encoded by viral gene gag: 22 and 17 amino acid residues in capsid and nucleocapsid proteins, respectively. Similar regions were found in potential protein products of ORF1. Another conservative region was revealed by multiple alignment in amino acid sequences of potential protein products of ORF1 of mobile elements and in p19 matrix protein of some retroviruses; this region is different in mobile elements from different classes, but within each class it shows high similarity. The secondary structure of this region is an alpha-helix.