This variant was seen as a compound heterozygote (with either P376PfsX30 or P469H) in two individuals out of a screen of 200 “so far unsolved” cases of congenital myasthenic syndromes. The authors mention screening 100 control DNAs (chromosomes?); we presume the mutation was not seen once in these.
This variant was seen compound heterozygously in 1 out of 18 families screened. The authors report “100 healthy controls were screened for each novel mutation” — because this variant was not novel (they note it was previously reported), this implies no controls were screened for the variant.