The specific locus mutagenesis test generated thirty overlapping deletions at the brown (Tyrp1) locus. This has permitted high resolution molecular and functional analysis of a 7-9cM interval on mouse chromosome 4. Complementation analysis revealed four new functional units in the central to distal part of this region, two of which are early embryonic lethals. This study has shown that one appears to result in neonatal death. The fourth, baf (brown associated fitness) has a sub-viable phenotype, demonstrating poor growth, possible gut abnormalities, nervous behaviour and death at around weaning age. This phenotype appears to be highly variable, with some homozygotes surviving to adulthood. A comparative study of the different deletions in their homozygous state has been undertaken to determine if these phenotypes or their severity are genetically separable. Generation of fine structure map of a 2.5cM region encompassing these genes containing over 30 markers has been completed. Techniques including sample sequencing, exon trapping, cDNA selection and homology mapping have identified transcripts from this region. This has resulted in a comparative transcript map of the mouse chromosome 4 interval and the conserved syntenic region on human chromosome 9. Several transcripts have been placed on this map and of particular interest is a transcript encoding 13 PDZ domains which maps distal to (Tyrp1). PDZ domains are known to interact with a number of proteins at specific cellular junctions and may be responsible for the sub-cellular localisation and clustering of proteins into functional complexes. There is evidence that the protein interacts with the C-terminus of the 5-HT2c receptor.