We have cloned and sequenced a cDNA from a human adult skeletal muscle cDNA library, encoding for a novel isoform of alpha-tubulin (tuba8) that is preferentially expressed in heart, skeletal muscle, and testis. A genomic DNA sequence from the chromosomal region 22q11 allowed us to determine the complete structure of the TUBAS gene that mirrors the canonical exon/intron organization of the vertebrate a-tubulin genes. We also cloned and sequenced the cDNA of its murine homologue (MMU-TUBA8). The latter encodes for a protein that differs from its human counterpart in only three amino acids, revealing an extreme rate of conservation that is even extended to both the 3' and 5' UTRs of the mRNAs. Sequence comparison of these novel isoforms with other known alpha tubulins shows that tuba8 is the most divergent member of the mammalian alpha-tubulin family. The sequence peculiarity of the human and murine tuba8 strongly suggests that they might have functional significance and, according to the multi-tubulin hypothesis, that they might play specific functional roles in the cytoskeleton.