1. Diagnosis of homocystinuria due to CBS deficiency 2. Capable of providing signed informed consent/assent and to comply with all study related procedures 3. Is ≥12 years of age (≥18 in the US) at the time of signing the informed consent/assent 4. Plasma tHcy >80 µM 5. Female subjects of child-bearing potential must have a negative serum pregnancy test during the screening period and a negative urine pregnancy test prior to dosing on the first day of treatment 6. If the subject (male or female) is engaging in sexual activity, he/she must be unable to become pregnant/cause pregnancy or must agree to use highly effective contraception 7. Willing to maintain a stable diet with no significant modifications while on study 1. Other medical conditions or co-morbidity(ies) that, in the opinion of the investigator, would interfere with study compliance or data interpretation (eg, severe intellectual disability that precludes completion of the required study assessments) 2. Currently participating in another therapeutic clinical study or has received any investigational agent within 30 days or 5 half-lives, whichever is longer, prior to the first dose of study drug in this study 3. Surgery requiring general anesthesia within 8 weeks prior to the first dose of study drug 4. Active infection requiring anti-infective therapy <2 weeks prior to the first dose of study drug in this study; anti-infective therapy that completes ≥2 weeks prior to first dose of study drug is acceptable 5. Pregnant or nursing 6. Females of child-bearing potential who are using or plan to use estrogen-containing contraception during the study 7. History of hypersensitivity to polyethylene glycol (PEG) that, in the judgment of the investigator, puts the subject at unacceptable risk for adverse events (AEs) 8. Serum creatinine level >1.5× the upper limit of normal (ULN) 9. Alanine aminotransferase (ALT), aspartate aminotransferase (AST), or total bilirubin level > 2× the ULN

Patients must have a confirmed diagnosis of LAL Deficiency. An Informed Consent and Authorization must be obtained prior to patient enrollment where required under applicable laws and regulations, or a waiver must be obtained by the Institutional Review Board/Independent Ethics Committee. Patients cannot be currently participating in an Alexion-sponsored clinical trial. Patients who have concluded participation in an Alexion-sponsored sebelipase alfa clinical trial are eligible to enroll in this Registry, and enrollment in the Registry will not exclude a patient from enrolling in a future clinical trial.