These diseases are caused mostly by autoantibodies to cell-surface or tissue-matrix molecules. This suggests that an important factor determining whether an autoantibody that crosses the placenta causes disease in the fetus or newborn baby is the accessibility of the antigen to the auto-antibody. Autoimmune congenital heart block is caused by fibrosis of the developing cardiac conducting tissue, leading to slowing of the heart rate (bradycardia), and there is evidence that this expresses abundant Ro antigen (see Section 13-9). Ro protein is a constituent of an intracellular small cytoplasmic ribonucleo-protein. It is not yet known whether it is expressed at the cell surface of cardiac conducting tissue to act as a target for autoimmune tissue injury.