Primary Myelofibrosis (PMF) or Post-Polycythemia Vera or Post- Essential Thrombocythemia Myelofibrosis (Post-PV/ET MF)

To provide extended access to momelotinib in 4 cohorts of subjects who are currently receiving treatment with MMB and have not experienced progression of disease: -Cohort 1:Study GS-US-352-0101, subjects with PMF or post-polycythemia vera/essential thrombocythemia myelofibrosis(post-PV/ET MF) -Cohort 2:Study GS-US-352-1214, subjects with PMF or post-PV/ET MF -Cohort 3:Study GS-US-352-1154, subjects with PMF or post-PV/ET MF -Cohort 4:Study SRA-MMB-301, subjects with PMF or post-PV/ET MF

Momelotinib Film-coated Tablets

Momelotinib

Film-coated

50mg, 100mg, 150mg, 200mg

Safety: Safety will be evaluated by the incidence and severity of AEs.

Inclusion Criteria
1) Currently enrolled in Studies GS-US-352-0101, GS-US-352-1214, or GS-US-352-1154, or SRA-MMB-301
2) Did not discontinue treatment with MMB for any reason while enrolled in Studies GS-US-352-0101, GS-US-352-1214, or GS-US-352-1154, or SRA-MMB-301
3) Negative serum or urine pregnancy test is required for female subjects of childbearing potential as described in Appendix 3
4) Male subjects and female subjects of childbearing potential who engage in heterosexual intercourse must agree to use protocol specified method(s) of contraception as described in Appendix 3
5) Any Grade 3 or 4 (Common Terminology Criteria for Adverse Events [CTCAE] Version 4.03) non-hematologic toxicity in the prior study that the investigator considers related to previous MMB use must have resolved, reverted to Grade 1, or reverted to baseline within the 30 days from the last MMB administration to Day 1 of this study
6) Any adverse event (AE) requiring MMB interruption during the prior study must have resolved, reverted to Grade 1, or reverted to baseline within the 30 days from last MMB administration to Day 1 of this study
7) Ability and agreement to attend protocol-specified visits at the study site
8) Able to comprehend and willing to sign the informed consent form

Exclusion Criteria
1) Known hypersensitivity to MMB, its metabolites, or formulation excipients
2) Incomplete recovery from major surgery prior to Day 1 of this study
3) Pregnant or lactating females
4) Presence of >/= Grade 3 (CTCAE Version 4.03) peripheral neuropathy
5) Known positive status for human immunodeficiency virus (HIV)
6) Known chronic active or acute viral hepatitis A, B, or C infection