Effect of Hepatic Impairment on the Pharmacokinetics of Mirdametinib

Inclusion Criteria (All Participants): * Participant understands the study procedures, is willing to comply with all study requirements and restrictions and agrees to participate in the study by providing written informed consent, prior to any study-related procedures being performed. * Participant is between 18 and 80 years of age (inclusive) at the time of informed consent. * Participant has a body mass index (BMI) ≥18 kg/m2 and ≤32 kg/m2 (inclusive) at Screening and Day -1 and a total body weight \>50 kg. * Male participants that agree to the following during the treatment periods and for at least 90 days after the last dose of study treatment: 1. Refrain from donating or preserving sperm; PLUS either 2. Be abstinent from heterosexual intercourse as their preferred and usual lifestyle (abstinent on a long term and persistent basis) and agree to remain abstinent. OR 3. Must agree to use a male condom when having sexual intercourse with women of childbearing potential (WOCBP). An additional form of contraception as described in Appendix 4 of the protocol should also be used by the female partner if she is of childbearing potential. Refer to Appendix 4 of the protocol for definition of WOCBP. * Female participants that are not pregnant or breastfeeding, and for whom one of the following conditions applies: 1. Refrain from donating or preserving eggs for at least 90 days after the last dose of study treatment PLUS either 2. Is a woman of non-childbearing potential, as defined in Appendix 4 OR 3. Is a WOCBP and agrees to use an acceptable contraceptive method as described in Appendix 4 from the time of informed consent and for at least 90 days after the last dose of study treatment; AND 4. WOCBP and post-menopausal women must have a negative serum pregnancy test at Screening and on CRU admission on Day -1 unless confirmed as surgically sterile. * Participant has sufficiently good venous access in at least one arm to confidently enable serial blood sampling. Inclusion Criteria (Participants with Hepatic Impairment): * Participant has chronic (\>180 days), stable hepatic insufficiency, with no acute episodes of illness or liver injury within 28 days prior to Screening due to deterioration in hepatic function and must remain stable through the Screening Period. * Concomitant medications for the management of an individual participant's medical history should be stable for at least 28 days prior to dose of study treatment. On a case-by-case basis, with approval from the Sponsor, participants receiving fluctuating concomitant medications and treatment may be considered if the underlying disease is under control and is not contraindicated by stated medication exclusions. * Aside from hepatic impairment, the participant must, in the opinion of the Investigator, be sufficiently healthy for study participation based upon medical history, physical examination, vital signs, 12-lead electrocardiogram (ECG), and screening laboratory evaluations. * Participant has normal or mildly impaired renal function as evidenced by estimated glomerular filtration rate (eGFR) determined by Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine formula (i.e., ≥ 60 mL/min/1.73 m2) or local standard. * Participant is willing and able to abstain from alcohol throughout the duration of the study (from Screening to FU phone call) and has a negative breath alcohol test at Screening and CRU admission (Day -1). * Participant may be enrolled if the criteria for moderate hepatic impairment based on Child-Pugh Classification Class B (Cohort 1) or severe hepatic impairment based on Child-Pugh Classification C (Cohort 3) is satisfied within 28 days prior to Day 1 and still at CRU admission (Day -1). Inclusion Criteria (Healthy Control Participants): * Participant is considered to be medically healthy, as determined by a responsible and experienced Investigator, based on a clinical evaluation (including medical history, physical examination, clinical laboratory tests, vital sign measurements, and a 12-lead ECG performed as directed in the SoA (Section 1.3), and the results of clinical chemistry, hematology, and urinalysis carried out at Screening and Day -1. * Participant is considered to be without hepatic disease and has normal hepatic function at screening and check-in (serum bilirubin, serum albumin, and international normalized ratio \[INR\]). Participants may be included if the alanine aminotransferase \[ALT\] and aspartate aminotransferase \[AST\] levels are below or within reference ranges. Participants will not be included if the ALT, AST, total serum bilirubin, and INR are outside the upper limit of normal per the local labs. In addition, participants will not be included if the serum albumin is below the lower limit of normal per the local labs. * If these tests are out of range, one laboratory repeat will be allowed at screening and upon check-in on Day -1. * Participant's age at Screening must be within 10 years of the Day -1 age for the intended paired participant. * Participant's BMI at Screening must be within 20% of the Day -1 BMI for the intended paired participant. * Participant has normal renal function as evidenced by eGFR as calculated by the CKD-EPI creatinine formula (i.e., ≥ 60 mL/min/1.73 m2) or local standard. Exclusion Criteria (All Participants): * Participant is deemed unsuitable for this study in the opinion of the Investigator for any additional reason, condition, or prior therapy. * Participant has clinically significant infections (e.g., coronavirus disease 2019 \[COVID-19\] or influenza) within 90 days prior to Day 1, as judged by the Investigator, or evidence of any infection with the past 14 days prior to Day 1. Chronic infection with Hepatitis B or C (\>180 days) may be eligible as judged by the Investigator in consultation with the Sponsor's medical monitor. If a participant tests positive for HIV at Screening, they are not eligible for participation in