Heterozygous trinucleotide in frame duplications, leading to expansions of variable lengths of a 20-alanine stretch (polyAla), is the most frequent PHOX2B variant associated with congenital central hypoventilation syndrome (CCHS), a rare neurocristopathy characterized by defective response of the autonomic nervous system to hypoxia and hypercapnia. Sequencing analysis has shown that the vast majority of polyAla expansions arise de novo; while in about 10% of cases, mutations are inherited by one parent who carries either constitutive or somatic mutations. To investigate transmission of PHOX2B mutant alleles from asymptomatic individuals, we have reassessed 44 parental pairs, previously resulted not to carry any mutation, by coupling amplification with FAM-tagged primers and capillary electrophoresis. Low levels of somatic mosaicism were shown in five parents previously undetected, thus increasing the inherited occurrence of the disease from 10% to 25% of the cases. Analysis of the technical detection limits has confirmed a power of resolution much higher for the "FAM" protocol than for the "sequencing" method. These observations are going to have relevant implications on how the carrier status of asymptomatic parents should be assessed and on successive genetic counseling to CCHS families.