The presence of evolutionarily conserved exon-intron block (exon 110nt — intron — exon 37nt) is the specific feature of nxf1 genes in Animalia. The intron in this block, which we call «cassette», can be retained in alternative transcripts of nxf1. The transcript with intron has a premature stop codon and has to be degraded by NMD, but it does not, because it was found in cytoplasm. The multiple alignment of cassette introns sequences of nxf1 genes allowed us to separate out different taxonomic groups [1]. Cassette inrons of nxf1 genes in mammals have four long evolutionarily conserved regions. The first one is 5'-end of cassette intron. It includes 17 complete codons that continue ORF of the preceding exon. The corresponding aminoacids form C-terminal end of supposed short protein. The second region contains CTE (constitutive transport element) that has been identified earlier by other authors. The third region forms a conserved hairpin in RNA secondary structure, so it can be a source of previously unknown n