Subjects must be monozygotic twins (MZ) as confirmed by twin-similarity questionnaire and by molecular cheek swab testing by polymerase chain reaction short tandem repeat multiplex loci sets to find differences in the DNA types of the two co-twins. The primary test set includes nine to fourteen unlinked loci with a combined matching probability for siblings p less than .0001. Cases yielding a probability of twinship less than 0.99 are tested further to a total of 21 loci. Subjects may also be requested to participate in an associated genetic protocol (96-M-0060).