We have recently cloned four novel human genes that encode the ancient conserved domain proteins (ACDP). The full-length cDNA sequence of ACDP1 consists of 5898 bp and encodes a predicted protein of 951 amino acids (AA). The transcript for ACDP2 has 4058 bp of cDNA sequence, encoding a protein of 875 AA. ACDP3 contains 3113 bp of cDNA sequence and encodes a putative protein of 707 AA. ACDP4 contains 4765 bp of cDNA sequence and encodes a protein of 775 AA. The ACDP genes belong to a highly conserved new gene family. The conserved region showed 62.8% of nucleotide sequence identity, and 65.5% of AA identity with 92% of AA homologies among ACDP members. The conserved domain is also found in genes from evolutionarily divergent species from bacteria, yeast, Caenorhabditis elegans, and Drosophila melanogaster to mammals. All ACDP genes except ACDP1 have a ubiquitous expression pattern while ACDP1 expression is restricted to the brain and testis. Immunofluorescence staining of premeablized HeLa cells showed that ACDP proteins are predominantly localized in the nucleus. Sequence homology analyses revealed AA property and structural homologies between the ACD domain and cyclin molecules.