We have sought to develop methodologies to identify genes that are preferentially expressed during the differentiation of mast cells from their hematopoietic stem cell precursors. By using a modified differential display protocol, we compared a subset of transcripts expressed in bone marrow cells differentiated into immature mast cells with the exogenous addition of stem cell factor (SCF) or interleukin 3. One gene was identified that was preferentially expressed in the SCF-derived cells and encodes a novel murine integrin beta subunit-like molecule, dubbed Pactolus-1 (Pactolus). Two distinct forms of Pactolus mRNA were detected which, via alternative splicing, are predicted to encode a membrane-bound form and truncated version of the protein. The full-length Pactolus gene product is very similar to a number of beta subunit integrin chains, particularly beta2, with the notable exceptions of the apparent deletion of the metal-binding site within the putative metal ion-dependent adhesion site-like domain of the Pactolus gene product and a cytoplasmic domain that shares no obvious homology to similar domains of the other beta subunit integrin proteins. Although the Pactolus sequence was first identified in immature mast cell samples, screening of murine tissues indicated the highest level of Pactolus expression was found in the bone marrow, suggesting that the expression of Pactolus is confined to immature and maturing bone marrow-derived cells, and that the SCF-derived mast cells are more representative of this state than are the interleukin 3-derived mast cells. Immunoprecipitation of Pactolus revealed a cell-surface protein with an apparent molecular mass of about 95 kDa. Surprisingly, no associating alpha integrin subunit could be identified suggesting that either Pactolus does not associate with another integrin subunit or the association is too weak to be identified. These data suggest that Pactolus represents a gene and gene product related to those of the integrin beta subunits but whose function(s) may be quite distinct from those of the integrin beta subunits.