The European Medicines Agency has required pre-treatment genotyping of CYP2C19 to determine the initial and maximum dosage, in order to avoid overexposure to mavacamten associated with a decrease in ventricular ejection fraction below 50% in slow metabolisers of CYP2C19 (AUC multiplied by 3.4). The study is a requalification for the search for DNA samples obtained during treatment in order to genotype the genes of interest.The primary objective of the study is to estimate, for each of the CYP2C19 phenotypes of interest determined by genotyping (ultra-rapid, rapid, normal/extensive and intermediate metabolisers), the proportion of patients who are non-responders to mavacamten at each time point (D0, Week 4, Week 8, Week 12 and Week 24 in the treatment of HCM). This is a multicentre (3 centres which are hospitals of APHP) study aiming to include 300 patients with obstructive hypertrophic cardiomyopathy treated with Mavacamten who underwent or are undergoing CYP2C19 genotyping at the start of treatment. The inclusion period is 36 months and the follow-up period is 6 months. The total duration of the study is 42 months.