In order to determine the molecular defect in the CFTR gene in uncharacterized CF patients from former Yugoslavia, we have employed SSCP analysis of PCR amplified fragments of exon 12 of the CFTR gene (Orita et al., 1989) (primer sequences available on request). A bandshift was detected in a DNA sample of a 2-year-old girl of Croatian origin. Sequence analysis in both directions (Sanger et al., 1977) identified an A 4 3 change at position 1838, leading to a change of Tyl--tCys in the CFTR protein (data reviewed but not shown). The mutation is located in the first nucleotide binding fold, in the p-D-sheet between loops 3 and 4, within the Walker B motif (Hyde et al., 1990). Mutations in the conserved Walker A and B motifs have been shown by functional analyses to alter the CFTR function (Anderson and Welsh, 1992). Furthermore, SSCP screening of exons 2, 3, 4, 5, 7, 8, 10, 11, 20, 21, and 23 of the CFTR gene in this patient did not reveal any additional sequence alteration. This mutation was found in 1 out of 62 non-AF508 CF chromosomes, 21 AF508 chromosomes, and 102 normal chromosomes. It segregates with an 11 (XV-~C, KM19) haplotype. The patient carried the AF508 mutation on the other chromosome. She was suffering from relapsing respiratory disorders.