The N terminus of SP140, between amino acids 29 and 157, is 49% identical to a corresponding N-terminal region of SP100. The C terminus of SP140 contains a cysteine-rich zinc finger motif and a bromodomain. SP140 also has a central nuclear localization signal. RNA dot blot analysis detected high SP140 expression in spleen and peripheral blood leukocytes, with much lower levels in thymus, prostate, ovary, small intestine, and colon; other tissues showed little to no expression. Low expression of a smaller transcript was also observed. Western blot analysis with PBC autoantibodies detected SP140 at an apparent molecular mass of 140 kD following in vitro transcription and translation. Immunohistochemistry demonstrated that SP140 colocalized with PML in dense nuclear bodies in activated myeloid precursor cells.