A Study to Test Whether Spesolimab Helps People With a Skin Disease Called Netherton Syndrome

Inclusion Criteria: * Male or female patients, aged 12 years and older (weight minimum is 35kg). * Confirmed diagnosis of Netherton syndrome (NS) (causative SPINK5 mutations) at baseline (Visit 2). * At least moderate severity of erythema at baseline (visit 2) (Ichthyosis Area Severity Index (IASI) score ≥ 16 and IASI-Erythema (E) score ≥8) and ≥ 3 on Investigator Global Assessment (IGA) score. * Signed and dated written informed consent and assent in accordance with International Council on Harmonisation-Good Clinical Practice (ICH-GCP) and local legislation prior to admission in the trial * Women of childbearing potential (WOCBP) must be ready and able to use highly effective methods of birth control per ICH M3 (R2) that result in a low failure rate of less than 1% per year when used consistently and correctly. A list of contraception methods meeting these criteria is provided in the clinical trial protocol (CTP) as well as in the patient, parent(s) (or patient's legal guardian) information. Exclusion Criteria: * Patients who have used topical corticosteroids (medium to high, US class I-V), topical retinoids, topical calcineurin inhibitors or keratolytics within 1 week prior to randomisation * Patients who have used emollient on the area to be biopsied in the previous 24 hours * Patients who have used systemic retinoids, other systemic immunosuppressants, systemic corticosteroids or phototherapy within 4 weeks prior to randomisation * Patients who have used systemic anti…