Some CFTR variants, mostly belonging tothe IV and V pathogenic variant classes, lead to a milder CF phenotype (Gan et al., 1995; Rozen et al., 1995; DeBraekeleer et al., 1997a ; Clain et al.,2005; Castellani and Assael, 2017).Previous genotype-phenotype correlations amongst the French-Canadian population haveidentified that amongst the F508del compound heterozygotes, the most common variantsin trans with F508del were c.579+1G>T (711+1G>T) (9%), c.489+1G>T(621+1G>T) (5%), A455E (1%), and L206W (1%), however, the most frequent isanother F508del variant (71%) (Rozen etal., 1992,1995). The gene discussed is CFTR; the disease is cystic fibrosis.