Study to Determine the Effectiveness of GSK1120212 in BRAF Mutation-positive Melanoma Previously Treated With or Without a BRAF Inhibitor

Inclusion Criteria:

* Metastatic cutaneous melanoma that was previously treated with: (Cohort A) a BRAF inhibitor either with or without other prior therapy. (Cohort B) at least 1 prior chemotherapy or immunotherapy, without treatment with a BRAF inhibitor.
* Documented positive BRAF mutation (V600E, V600K, or V600D).
* Subjects must provide archived tumor tissue or undergo fresh tumor biopsy prior to enrollment.
* The subject must have a radiographically measurable tumor.
* The subject is able to carry out daily life activities without significant difficulty (ECOG performance status score of 0 or 1).
* Able to swallow and retain oral medication.
* Sexually active subjects must use acceptable methods of contraception during the course of the study.
* Adequate organ system function and blood cell counts.

Exclusion Criteria:

* The subject has had major surgery or received certain types of cancer therapy within 21 days before starting the study.
* Previous treatment with a MEK inhibitor.
* Current use of a prohibited medication listed in the protocol.
* Uncontrolled glaucoma.
* Brain metastasis, unless previously treated with surgery or stereotactic radiosurgery, and the disease has been stable for at least 2 months prior to enrollment.
* Current severe or uncontrolled systemic disease.
* History of clinically significant heart, lung, or eye/vision problems.
* Significant unresolved side effects from previous anti-cancer therapy.
* The subject is pregnant or breastfeeding.