We have cloned and expressed in vaccinia virus a cDNA encoding an ubiquitous 501-amino-acid (aa) phosphoprotein that corresponds to protein IEF SSP 9502 (79,400 Da, pI 4.5) in the master 2-D-gel keratinocyte protein database [Celis et al., Electrophoresis 14 (1993) 1091-1198]. The deduced aa sequence contains 9 Trp residues, some of which are localized in repeats and that characterise the protein as a member of the WD-40 family, a group of proteins having 40-aa repeats containing Trp and Asp [Duronio et al., Proteins 13 (1992) 41-56; Van der Voorn and Ploegh, FEBS Lett. 307 (1992) 131-134]. The protein contains a nuclear targeting signal (KKKGK), and fractionation of transformed human amnion cells (AMA) in karyoplasts and cytoplasts confirmed that it is predominantly localized in the nucleus. Database searching indicated that IEF SSP 9502 is a putative human homologue of the Saccharomyces cerevisiae periodic Trp protein, PWP1, a polypeptide that may play a regulatory role in cell growth and/or transcription.