Cystic fibrosis (CF) and primary ciliary dyskinesia (PCD) are hereditary lung diseases. Cystic fibrosis is a monogenic disease, while PCD has a genetically heterogeneous nature and is 2 times less common than CF. More than 50 genes are responsible for PCD, one of them being the DNAAF11 gene (LRRC6 according to the previous nomenclature) – the assembly factor of the external and internal dynein arms. When the mutation is present, components of the dynein arms remain in the cytoplasm of cells. The aim is to present a clinical case of a newly diagnosed PCD in a 44-year-old woman with a homozygous pathogenic variant of the nucleotide sequence NM_012472.6:c.436G>C in the DNAAF11 gene. This clinical case demonstrates the complexity of verification and differential diagnosis of cystic fibrosis and primary ciliary dyskinesia, the features of the disease, and the modern diagnostic capabilities. The difficulties and some typical mistakes in the management of such patients are presented. Conclusion. The presented clinical case illustrates the difficulties of PCD diagnosis. It demonstrates that such patients need a comprehensive examination for a thorough differential diagnosis to exclude diseases with a similar clinical picture. The long-term follow-up is carried out by a multidisciplinary team of specialists and must include microbiological monitoring.

The DNAAF11 gene, also known as LRRC6, follows autosomal recessive inheritance and plays a key role in the assembly of dynein, a protein necessary for the normal functioning of cilia. Mutations in this gene can lead to primary ciliary dyskinesia (PCD), a rare hereditary disease characterized by impaired function of the ciliated epithelium, primarily in the respiratory system but also in other organs. Defects in dynein arms associated with mutations in the DNAAF11 gene disrupt their rhythmic movement, leading to mucus stagnation, chronic inflammatory processes, and increased susceptibility to respiratory tract infections. The aim of this work is to describe the clinical case of a family consisting of a mother and her son, both with a confirmed diagnosis of PCD. The study revealed a mutation in the DNAAF11 gene in both patients: the mother’s was in a homozygous state (NM_012472.6: c.436G>C), and her son’s was in a compound heterozygous state: one NM_012472.6: c.436G>C variant inherited from the mother (not previously described), and the second, NM_012472.6: c.1011A>G, inherited from the father. The child’s diagnosis was confirmed by segregation analysis. Methods. The PCD diagnosis included: molecular genetic analysis, segregation analysis, video microscopy, electron microscopy of the ciliated epithelium, air-liquid interface (ALI) cell culture, and immunofluorescence staining. Conclusion. This clinical case highlights the importance of identifying genetic relationships and features of PCD within families. The newly discovered mutations expand the spectrum of variants in the DNAAF11 gene associated with ciliary defects in PCD and emphasize the need for molecular genetic studies. Early diagnosis contributes to timely treatment initiation, preventing disease progression and improving patients’ quality of life.