The specificity of the ubiquitin degradation system is regulated through interaction between individual ubiquitin-conjugating enzymes (E2s) and multiple ubiquitin-protein ligases (E3s). Here we describe the characterisation of a novel gene (ARIH1) that encodes the human homologue of Drosophila ariadne which interacts with the E2s, UbcH7 and UbcH8 and represents a component of an E3 complex. Three PACs (189N19, 142P17 and 179H7) were isolated that contain this gene. Using these PACs as probes, we mapped ARIH1 to human chromosome 15q24 by fluorescence in situ hybridisation (FISH). Sequencing of the ARIH1 PACs showed that the gene has 13 introns. In addition, we isolated two PACs (345D8 and 571P19) containing the mouse orthologue (Arih1) of ARIH1. The intron-exon structure of Arih1 was identical to ARIH1 and the proteins demonstrated a 98% identity at the amino acid level. Furthermore, comparison of Drosophila ariadne with ARIH1 indicates an identity at the amino acid level of 70% and introns at 3/7 identical sites. The high degree of homology demonstrated by the mouse and human orthologues of Drosophila ariadne indicates an important, conserved biological function, consistent with a putative role in ubiquitylation. Copyright 2000 S. Karger AG, Basel.