To the Editor.—The recent communication by Natelson et al1at the first glance appears to add another report to the compendium of chromosomal abnormalities, specific or nonspecific, as the case may be. The nonspecific chromosomal abnormalities probably are not significant since they may not have any relation to the clinical nature or etiology of the disease. Indeed the presence of the same chromosomal abnormality (deletion of E-18 chromosome) in the normal father, propositus with polycythemia vera, and mentally retarded sister almost suggests that the chromosomal abnormality is an incidental finding. On closer examination of the karyotypes, particularly the variation in the size of "deleted E-18," one wonders if the karyotypes of the propositus and his father could not have been rearranged. For example, the karyotype of the patient (Fig 11(p243)) could be interpreted as that of normal male by pairing one of the members of pair 15