The primary objective of the study is to retrospectively investigate changes in lymphocyte counts and lymphocyte subtypes, with a focus on CD4+ and CD8+ T cells, in participants on Tecfidera therapy for at least 6 months. The secondary objective is to investigate changes in lymphocyte subtypes other than CD4+ and CD8+ T cells.

Key Inclusion Criteria:

• Initiated Tecfidera treatment for the first time on or after 27 March 2013 and received at least 6 months of continuous treatment with Tecfidera
• Clinical diagnosis of a relapsing form of MS
• A baseline measurement for ALC and absolute CD4+ or CD8+ count within 6 months prior to Tecfidera initiation
• At least 1 measurement for ALC and absolute CD4+ or CD8+ count while on Tecfidera therapy for at least 6 months

Key Exclusion Criteria:

• Clinical diagnosis of human immunodeficiency virus (HIV) or acquired immunodeficiency syndrome (AIDS) prior to Tecfidera initiation
• Participation in DEFINE 109MS301 (NCT00420212) or CONFIRM 109MS302 (NCT00451451)

NOTE: Other protocol-defined inclusion/exclusion criteria may apply.

This study will be conducted in patients with relapsing forms of multiple sclerosis (MS) who initiated Tecfidera treatment for the first time (treatment naïve) under routine clinical care. For inclusion in the study, patients' charts must have a baseline measurement for absolute lymphocyte count (ALC) and absolute CD4+ or CD8+ count within 6 months prior to Tecfidera initiation, and at least 1 measurement for ALC and absolute CD4+ or CD8+ count while on Tecfidera treatment for at least 6 months. For patients who discontinue Tecfidera, data will be collected for up to 6 months following Tecfidera discontinuation.