Serum cholinesterase (pseudocholinesterase) in Down's syndrome: 1. Phenotype frequencies at the E1 and E2 loci.

Dibucaine, fluoride and RO2-0683 inhibition studies were used to determine the serum cholinesterase (pseudocholinesterase) phenotypes at the E1 locus in a sample of 130 subject with Down's syndrome and fifty-three mentally retarded control subjects. No example of the Ef1 and Es1 allele was detected in either group, nor were any of the genotype E1a E1a detected. The gene frequency of E1a for the control group (0.0189) resembles closely that reported in the literature for normal European populations but in the Down's group was significantly lower (0.0038). The means and distribution of the dibucaine, fluoride and RO numbers in both groups were similar to those reported in normal subjects. The presence of the additional C5 cholinesterase type heterozygous for a variant cholinesterase on the E2 locus) was detected after starch gel electrophoresis, and the frequency was found to be raised in both groups. Several possible environmental factors (age, sex, maternal age, etc.) were investigated to account for this finding but with no success.