Cotton leaf-curl disease - CLCuD - is of major economic importance in cotton-growing areas of the far-east. The infectious agent appears to be a single-stranded DNA molecule of approx 1350 nucleotides in length, which, when inoculated with the Begomovirus into cotton, induces symptoms typical of CLCuD. This molecule requires the Begomovirus for replication and encapsidation. DNA beta encodes a single protein, betaC1. The intracellular distribution of betaC1 is consistent with the hypothesis that it has a role in transporting the DNA A of Begomovirus from the nuclear site of replication to the plasmodesmatal exit sites of the infected cell. The DNA beta-encoded protein, betaC1, is the determinant of both pathogenicity and suppression of gene silencing.