Turner syndrome affects 1/2500 female newborns. It is characterized by a short stature, gonadal dysgenesis and bone anomalies. It is secondary to X chromosome abnormality. The clinical course can be marked by various complications, including degeneration of gonadal streaks into cancer (gonadoblastoma). The risk of gonadoblastoma is increased by the presence of Y chromosome, with a risk of 19 to 43%. However, Y chromosome material may be difficult to identify due to its mosaic state, at varying rates depending on the tissue. Free circulating DNA (cfDNA) corresponds to fragments of extracellular DNA present in the plasma, released into the circulation during cell death processes by the various tissues of the body. Due to its multiple tissue origins and easy collection, cfDNA appears to be a suitable matrix for searching for low mosaic Y chromosome sequences in patients with Turner syndrome. The main objective of the study is to develop a cfDNA-based test to look for Y chromosome sequences in 50 patients with Turner syndrome. The secondary objectives are to determine the mosaic detection threshold of this test and to compare the performance of this test with the fluorescence in situ hybridization (FISH) technique used in routine diagnosis. This study will assess the detection sensitivity of this test and its relevance in a clinical context.