Chromosomal DNA Cytophotometry in 20q—Nonspecific Myeloid Disorders'

DNA cytophotometry was used to quantify the chromosomal alterations in the bone marrow and blood of three patients with nonspecific myeloid disorders. All patients possessed a popu lation of cells with a morphologically abnormal chromosome 20, del(2OXqll). In two of the patients, the abnormal chromo some 20 showed nearly identical DNA measurements with a net loss of 0.37% of the total autosomal DNA in one patient and 0.38% in the second. The third patient had a net loss of only 0.25% of the autosomal DNA. Analysis of the DNA content of the long arm and short arm of the abnormal No. 20 indicated that all three cases had chromosomal material added to the short arm (0.1 0 to 0.14% of the autosomal DNA). About 0.50% of the autosomal DNA was deleted from the long arm in two of the patients; only 0.35% of the autosomal DNA was deleted from the long arm in the third case. Within the limit of resolution, there is no evidence that the material lost has been translocated intact to another chromosome. The origin of the 20q —chro mosome as the result of an incomplete pericentric inversion is suggested.