This study is a multicenter, single-arm, open-label phase II study to assess the efficacy and safety of YH003 in combination with pembrolizumab and nab-paclitaxel in the first-line treatment of patients with unresectable/metastatic mucosal melanoma.

Inclusion Criteria:

• 1.Subjects must have the ability to understand and willingness to sign a written informed consent document.
• 2. Subjects must have histologically advanced or cytologically confirmed metastatic or unresectable mucosal melanoma;
• 3.Subjects have not received standard systemic therapy; patients have disease progression 6 months or more after the end of neoadjuvant or adjuvant therapy (except nab-paclitaxel), and can be enrolled in the clinical study;
• 4. Subject must have at least 1 unidimensional measurable disease by RECIST 1.1;
• 5. Subjects must be age 18 years or older;
• 6. Subjects must have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
• 7. Life expectancy ≥3 months based on investigator's judgement；
• 8. Subjects must have adequate organ function;
• 9. Women of childbearing potential need to have a negative pregnancy test and need to take contraceptive/contraceptive measures including their partners.

Exclusion Criteria:

• 1.Subjects have another active invasive malignancy within 5 years；
• 2.The subject has received anti-tumor therapy or other investigational drug therapy or traditional Chinese medicine (herbal medicine) with anti-tumor indications prior to the first dose;
• 3.Subjects with a history of ≥ Grade 3 immune-related adverse events resulted from previous immunotherapy.
• 4.History of clinically significant sensitivity or allergy ；
• 5.Primary central nervous system (CNS) malignancies or symptomatic CNS metastases.；
• 6.History of (non-infectious) pneumonitis that required corticosteroids or current pneumonitis, or history of interstitial lung disease；
• 7.Subjects have active pulmonary embolism with hemodynamic changes 12 weeks before the first dose;
• 8. Subjects must not have a known or suspected history of autoimmune disease within 3 years prior to the first dose of study treatment;
• 9. Subjects have clinically uncontrolled diseases;
• 10. Subjects have severe cardiovascular disease;
• 11. Subjects have evidence of active infection;
• 12. Subjects must not have a known or suspected history of an autoimmune disorder；
• 13.Major surgery within 4 weeks prior to study entry and Minor surgery within 2 weeks prior to the first dose.
• 14. Any condition that the investigator assesses as inappropriate for participation in the study.