To the Editor.
—Studies of twins can be especially illuminating with respect to the onset and progress of disease. A case report of the first pair of identical twins, concordant for both mitral valve prolapse and ischemic cerebrovascular disease was presented by Fisher and Budnitz1 in the November Archives. Unfortunately, the investigators failed to record the methods by which the monozygosity of this pair was established. We are aware that experienced twin investigators seldom make errors in the determination of zygosity. Investigators unfamiliar with the work of the authors and future investigators will, however, hesitate to pool this case with others, thus impeding the systematic accumulation of useful information.Objective classification of twin pairs is based on co-twin comparisons across factors comprising the eight major RBC systems. Additional comparisons of anthropometric characteristics, such as fingerprint ridge count, can further improve accuracy.2 These procedures are extremely critical in twin