Amino acid D233 is crucial to a dominant epitope recognized by sera from mothers of children with congenital heart block (CHB). (a) Schematic view of p200 peptides containing point mutations (alanine scan). The amino acid substituted by an alanine is indicated in black. (b) Reactivity of CHB mother sera [n = 18, mean ± standard error of the mean (s.e.m.)] to mutated p200 peptides described in (a).