We have previously isolated from a 1246 adipocyte cDNA library a cDNA clone called 154, corresponding to a mRNA that increases abundantly at a very early time during the differentiation of 1246 adipocytes and in adipocyte precursors in primary culture. We show here that the mRNA encoded by this cDNA is expressed abundantly and preferentially in mouse fat pads. A full-length cDNA for clone 154 was isolated by the RACE (rapid amplification of cDNA ends) protocol. Sequence analysis of this cDNA indicates that it encodes a protein of the 425 amino acids [tentatively named adipose differentiation-related protein (ADRP)] that does not have any similarity with sequences contained in the GenBank DNA and Protein Identification Resource protein data bases. Immunoblot of 1246 cell extracts with an antibody raised against the expressed ADRP shows that the 1246 cells contain a 50-kDa protein, the production of which increases as the cells differentiate. Localization of ADRP in 1246 cells indicates that ADRP is absent from nuclear and cytosolic fractions and is found as a membrane-associated protein. These results demonstrate that adipocyte differentiation is accompanied by early expression of a mRNA encoding a membrane-associated adipose differentiation related protein that is adipose tissue specific in vivo.