Inclusion Criteria:

- The subject has a diagnosis of polycythemia vera (PV), and has failed, or is
intolerant of, standard therapies or refuses to take standard medications.

- The subject is ≥18 years old.

- The subject has an Eastern Cooperative Oncology Group (ECOG) performance status of
≤2.

- The subject has adequate organ function.

- Subjects who have received phlebotomy due to PV must have documented phlebotomy
history for 12 weeks prior to enrollment.

- The subject has the capability of understanding the informed consent document and has
signed the informed consent document.

- Sexually active subjects (male and female) must use medically acceptable methods of
contraception during the course of the study.

- Female subjects of childbearing potential must have a negative pregnancy test at
screening.

- The subject has had no other diagnosis of malignancy or evidence of other malignancey
for 2 years prior to screening for this study (except non-melanoma skin cancer or in
situ carcinoma of the cervix).

Exclusion Criteria:

- The subject has received treatment for PV within 14 days prior to first dose of XL019

- The subject has uncontrolled intercurrent illness including, but not limited to,
ongoing or active infection, hypertension, symptomatic congestive heart failure,
unstable angina pectoris, myocardial infarction within 3 months, or cardiac
arrhythmias.

- The subject is pregnant or breastfeeding.

- The subject is known to be positive for the human immunodeficiency virus (HIV).

- The subject is unable or unwilling to abide by the study protocol or cooperate fully
with the investigator or designee.