∙ Screening Cohort (US patients only):

• Age ≥18 years when informed consent is obtained

• Has freely given written informed consent to participate in the study

∙ Treatment Cohorts (Cohorts 1 and 2):

• Positive for anti-HTLV-1 antibody in the serum using an FDA approved assay for US patients (Avioq HTLV-I/II Microelisa System). UK patients should use UK Accreditation Service (UKAS) accredited tests, Abbot Architect ELISA Serology Screening assay and confirmatory serology Western Blot (performed at Public Health England, Virus Ref Dept, Colindale).

• High-risk phenotype (PVL≥8% of PBMC)

• Age ≥18 years when informed consent is obtained

• Primary organ functions are stable

‣ Neutrophil count: ≥ 1000/mm3, unless patient has diagnosis of ethnic neutropenia

⁃ Platelets: ≥100,000/mm3

⁃ Hemoglobin: ≥9.0 g/dL

⁃ Serum aspartate aminotransferase (AST): ≤1.5x upper limit of normal (ULN)

⁃ Alanine aminotransferase (ALT): ≤1.5x ULN

⁃ Total bilirubin: ≤1.5x ULN

⁃ Serum creatinine (Cr): ≤1.5x ULN

⁃ Blood oxygen saturation (SpO2): ≥90%

• Electrocardiogram (ECG): No abnormal findings requiring treatment are observed

• Has freely given written informed consent to participate in the study

• For females of reproductive potential: use of effective contraception during treatment and for at least 3 months after completion of mogamulizumab therapy. For males who have sexual intercourse with females of reproductive potential: use of effective contraception during treatment and for at least 3 months after completion of mogamulizumab therapy.