You are an expert at extracting action from text. Your task is to extract the most important action from the following experimental procedure.
Requirements:
1. The output format should have only one verb word.
2. Please extract the verb word from the given procedure without creating your own summaries.
3. Verb should be in uppercase and origin form.
4. If there is no action to extract, use NONE instead

Here are some examples of the output format.

Procedure:
Maintain the cells for 72 h in the incubator at 37°C and 5% CO2.
Answer:
MAINTAIN

Procedure:
Turn off the magnetic stirrer and apply a voltage of 20 V. Maintain this state for 4 min.
Answer:
TURN

Procedure:
T cell exhaustion assay 1
Answer:
NONE

The given experimental Procedure is:
---SENTENCES---
Answer: