The SON gene, which maps to human chromosome 21q22.1-q22.2, encodes a novel regulatory protein. Here we describe the organization of the Son locus in the mouse genome. The mouse Son gene spans a region of approximately 35 kb. The coding region is more than 8 kb in length and has been completely sequenced. The gene is organized into 11 coding exons and 1 noncoding 3'UTR exon, with over 70% of the coding region residing in one 5.7-kb exon. The gene contains at least one alternative exon, N/C exon 1, which can be used, by splicing, to generate a truncated form of the SON protein. Further investigation of the mouse Son locus has identified the genes directly flanking Son. The glycinamide ribonucleotide formyltransferase gene, Gart, is encoded 5' of Son in a head-to-head arrangement, with the start of both genes lying within 899 bp. Sequence comparison with the expressed sequence tagged database identified a novel gene within 65 bp of the 3' end of Son, which we have named Donson. In this unusually compact gene cluster, we have found overlap in the pattern of expression between Gart, Son, and Donson. However, at least two of these genes have very different functions. While GART is involved in purine biosynthesis, we find that SON shows the characteristics of "SR- type" proteins, which are involved in mRNA processing and gene expression.