IN previous cornrnunicat. ions we reported that the proportion of aneuploid cells in cultures of human leucocytes increased with the age of the subject" and that, at any given age among adults, the proportions did not differ appreciably between persons with and without cancel". We now have much more data, and also have evidence that t, here is a difference between the sexes both in the manner in which the proportions of aneuploid cells change with age, and also in regard to the chromosomes affected.

Altogether data are available for 8,380 cells from 247 subjects with normal karyotypes inclusive of thmle reported previously. These cells have been obtained from blood cultures prepared by the technique of Moorhead et aZ.". For each aneuploid cell an attempt has been made to describe the missing or additional chromosomes. The distribution of chromosome counts is recorded in Table 1 for each age-group, and divided according t, o chromosomal sex. The proportion of aneuploid cells (expressed as a percentage of the diploid cells) inereases with age in both sexml; but the rate of inerease in the proportion of hypodiploid cells differs between the sexes. Comparison of the regression coefficients in the linear regression formulre (Table 1) shows the coefficients for men and women to differ significantly (P< 0· 01). Furthermore the male data are satisfactorily explained by a linear regression, whereas inspection suggests that the female data would be better fitted by a eubic curve in which the rate of increase is low under the agc of 45 years, high between the ages of 45 and 64 years, and low again at ages over 65 years. Under 45 years of age, the trend in the