A Study of LY3039478 in Combination With Dexamethasone in Participants With T-ALL/T-LBL

Inclusion Criteria:

- Have acute T-cell lymphoblastic leukemia (T-ALL) or T-cell lymphoblastic lymphoma
(T-LBL).

- T-ALL or T-LBL participants with relapsed/refractory disease.

- Have had at least 60 days between prior hematopoietic stem cell transplantation (SCT)
and first dose of study drug.

- Have a performance status of 0 to 2 on the Eastern Cooperative Oncology Group (ECOG)
scale for adults.

- Lansky score >50% for participants <16 years old.

- Have adequate organ function.

- Are at least:

- adult Phase 1 Part A and Phase 2: ≥16 years old at the time of screening

- pediatric Phase 1 Part B: 2 to <16 years old

- Men and women with reproductive potential: Must agree to use a reliable method of
birth control during the study and for 3 months following the last dose of study
drug(s) or country requirements, whichever is longer.

- Females with childbearing potential: Have had a negative serum pregnancy test ≤7 days
before the first dose of study drug and also must not be breastfeeding.

- Are able to swallow capsules and tablets.

Exclusion Criteria:

- Have previously completed or withdrawn from this study or any other study
investigating LY3039478 or other Notch inhibitors.

- Have evidence of uncontrolled, active infection <7 days prior to administration of
study medication.

- Have current or recent gastrointestinal disease with chronic or intermittent diarrhea,
or disorders that increase the risk of diarrhea, such as inflammatory bowel disease.

- Have active leukemic involvement of the central nervous system (CNS).