The Authors studied the leukocyte alkaline phosphatase (LAP) activity in 40 subjects with Down's syndrome. The age of the patients ranged from one week to 15 years. Chromosome analysis was carried out in 13 cases. 42 normal children of the same age as the patients were used as controls. The mean value of LAP in the patients with Down's syndrome was 175.5 and in the control group 108.8. In the latter group the LAP activity was clearly influenced by the age; the highest values were obtained in children under 11 years of age. The regression coefficient of the LAP on the age in the control group was –3.73. In the patients with Down's syndrome the differences observed between the various age groups were not statistically significant.
Chromosome studies showed a 21 trisomy in 12 cases; in one case a 15/21 translocation was found. The latter case presented an elevated LAP activity.
The Authors support the hypothesis that the gene or genes controlling LAP activity are located in the 21 chromosome and that the LAP activity may vary under the influence of several factors within the limits of the genetic control.