In a cytogenetic study on the residents in the high level natural radiation areas of Ramsar, a woman had an acentric like C group chromosome showing the premature centromere separation in the peripheral lymphocytes. This unusual chromosome was identified as an X chromosome with Fluorescent In situ Hybridization (FISH) method. Analysis of the 2038 cells revealed that these unusual X chromosomes had ability to replicate. This replication was associated with non-disjunction leading to aneuploid cells. In this paper, induction of the premature centromere separation (PCS) in the aged women is discussed in relation to the effect of the chronic exposure to low dose.