One of the features of intron-containing genes of the nxf (nuclear export factor) family in different organ- isms is the presence of an evolutionarily conserved exon-intron block: exon 110nt-intron-exon 37nt. The intron in this evolutionarily conserved block, which we call a “cassette” intron, can be excised or retained in alternative transcripts of nxf1. It corresponds to intron 10 - 11 in the genes that are orthologous to nxf1 in vertebrates, and intron 5 - 6 in the genes that are orthologous to nxf1 in Drosophilidae. The align- ment of sequences of cassette introns in nxf1 genes in vertebrates has revealed four evolutionarily con- served sequences: 1) 5’ flanking sequence, 2) a region containing СТЕ (constitutive transport element), 3) third conserved sequence, and 4) 3’ flanking sequence. Introns 5 - 6 of nxf1 in Drosophilidae have no similar conserved sequences. The results of sequence align- ment demonstrate a similarity between cassette in- trons of nxf1 in Drosophilidae in two poly(A) se- quences. The