We have identified a callus associated protein (CAP) as a new molecular marker for proliferative growth and early cellular differentiation in higher plants. Antiserum directed against theSorghum callus associated protein (CAP) was employed to isolate a cDNA clone (CAP-C2) from an expression library prepared from mRNA from actively dividing cell suspension cultures ofNicotiana tabacum L. cv. xanthi. The derived amino acid sequence contained structural features of eukaryotic transcriptional factors, including three putative zinc fingers, three activator domains, and a nuclear localization signal. A fusion protein formed between part of the CAP-C2 polypeptide andEscherichia coli beta-glucuronidase specifically accumulated within the nuclei of transfected protoplasts, suggesting a nuclear localization for the CAP protein. Increased levels of expression of CAP in vivo were observed in actively proliferating tobacco tissues, whereas expression in vitro was induced when mature, differentiated explants proliferated into callus or cell suspensions. A 6.2 kb unstable mRNA and two low-abundance nuclear proteins (p66 and p68) containing epitopes encoded by CAP-C2 accumulated in plant tissues undergoing proliferative growth but were absent in mature, differentiated tissues. The corresponding genes were also present inNicotiana sylvestris andPetunia hybrida