Original: While normally rare, achromatopsia is very common on the island of Pingelap, a part of the Pohnpei state, Federated States of Micronesia, where it is called maskun: about 1/12 of the population there has it.
Simplified: It occurs almost only on the island of Pingelap, a part of the Pohnpei state in the Federated States of Micronesia. There it is called maskun: about one person in 12 of the population has it.