Is the sentence grammatically okay? Or flawed?

...deem the main mechanism to be...

or

...deem that the main mechanism is....

So? The meaning would be totally different, McTag.

The researchers got a fact from their observation - that fact is that " the special hormone increases the breast cancer risk ".

To investigate the mechanism behind the fact, they further found that the mechanism is that it facilitates cell proliferation.

You think so?

The verb 'to deem' means to decree or state or believe that something is the case, usually in a defined situation. In the event of the death or incapacitation of the President of the United States, the Head of State is deemed to be the Vice-President. For the purposes of a hypothesis, if something is deemed to be the case, that means that the hypothesis holds or advances the proposition that the stated thing is the case.

Today's hypotheses deem the major mechanism whereby the special hormone increases breast cancer risk to be its facilitation of cell proliferation.

in the event

Thank you for spotting my typing error which I have now corrected.