The goal of this observational study is to learn the effects of the drug velmanase alfa (Lamzede®) in the bodies of children under the age of 3 with Alpha-Mannosidosis.

The main questions it aims to answer are:

• study the effect of velmanase alfa on a marker of the disease called GlcNAc(Man)2 after one year of therapy
• explore how the child's body reacts to velmanase alfa during the therapy The parents or legal guardians of participants will be asked to provide the results of analyses performed in the routine clinical setting related to the participant's general health and the administration of velmanase alfa.

Additional data will be extracted from other observational sponsored studies/registries, compassionate use programs, investigator-initiated studies (IIS), and published case reports (presented in the literature) if existing.

Inclusion Criteria:

• Subjects with the provision of informed consent from their legal guardians (LAR)
• Have a confirmed diagnosis of alpha mannosidosis
• Have initiated treatment with velmanase alfa between birth to at least six weeks before turning 3 years of age
• Have information on the disease marker GlcNAc(Man)2 obtained:

before velmanase treatment initiation (ideally max 6 month before), and at least one post-treatment sample, collected following at least six weeks of treatment.

- Participants treated with Lamzede, 1 mg/kg body weight, via weekly intravenous infusions.

Exclusion Criteria:

Participants who have undergone prior hematopoietic stem cell transplantation (HSCT) or other investigational therapies for treating alfa mannosidosis (supportive treatments acceptable).

Paediatric patients with a confirmed diagnosis of alpha mannosidosis via MAN2B1 genetic testing and/or alpha-mannosidase activity in leukocytes or fibroblasts < 10% of normal activity, with data for at least one pre- and one post-Lamzede treatment sample obtained when < 3 YOA, after at least 6 weeks of treatment (primary analyses).