This is a phase II intervention to propose a new melanoma chemoprevention agent. The investigators believe oxidative stress/damage in nevi is a probable indication for melanoma risk, and propose that reduced melanoma risk in humans can be inferred by protection of nevi from ultraviolet light (UV)-induced oxidative changes. The investigators will 1) evaluate whether administration of NAC around the time of UV exposure will reduce melanoma risk in high-risk patient populations with genetic susceptibility to UV-induced oxidative stress, and 2) examine key genetic variants that will identify which individuals are most likely to benefit from chemoprotection.

Inclusion Criteria:

• Must have at least 2 nevi (each >6 mm diameter) not clinically suspicious for melanoma that can be biopsied.
• Must be able to receive informed consent and sign an approved consent form that conforms to federal and institutional guidelines.

Exclusion Criteria:

• The patient is a minor (< 18 years old).
• The patient cannot speak/understand English or Spanish. (NOTE: A Spanish consent form and certified interpreter can be made available if needed)
• The patient is pregnant. (NOTE: All female patients who have not had a hysterectomy and are not post-menopausal (i.e. post-menopausal for 1 year and not of child-bearing potential) will have a urine pregnancy test.)
• The patient is a prisoner, critically or mentally ill, or otherwise incapacitated or considered vulnerable.
• The patient has history of allergic reaction to NAC.
• The patient has history of severe asthma.
• The patient has been taking NAC or any other oral antioxidant.
• The patient has recent history (i.e., 3 months) of sunless tanning (tanning bed) or extensive sunburn.