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The phrase "expression fell" is correct and can be used in written English.
It is typically used to describe a change in someone's facial expression or the way they are conveying their emotions. Example: As soon as he heard the news, his usually cheerful expression fell, and his face became solemn.
Exact(31)
Similarly to MCF7 cells, PR expression fell dramatically following 2 days of estrogen deprivation, was no longer expressed after 2 3 weeks, and remained undetectable on the protein level for the remainder of the study.
When he got in the driver's seat, a black expression fell over his face.
During the first 8 hours of exposure to simulated 0 × g, PTHrP expression fell precipitously, decreasing by 80 90%; during the subsequent 64 hours, PTHrP expression remained at this newly established level of expression.
Compared to pre-transplant levels, IL-4 gene expression fell significantly on post-operative days 2 and 7 before returning to baseline values by day 14 in the non-rejectors.
During the 1960s the original expression fell out of fashion in favour of the more official-sounding "unidentified flying object", a phrase borrowed from the US Air Force.
However, the word Universe was understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies.
Similar(29)
As ENG is expressed in undifferentiated ES cells and expression falls during ES differentiation (supplementary material Fig. S1) forced expression of ENG prior to the emergence of FLK1+ mesoderm probably skews differentiation towards cell fates other than those derived from progenitors that give rise to blast colonies.
Kerry is tall and lean, with carefully coiffed dark hair, a sharp nose and chin, and a mouth that seems small for his face, which perhaps explains why his expression falls into a smile only with reluctance.
Pfeiffer — who admirably carried off the exotic roles of Madame de Tourvel in "Dangerous Liaisons" and a Soviet damsel in distress in "The Russia House" — makes no attempt at the "trailing slightly foreign accent" Wharton ascribes to Ellen Olenska; instead, she does a kind of Grace Kelly imitation and in moments of urgent expression falls into the laid-back cadences of her native California.
Next, some are local, some are global (albeit on some accounts only accidentally global, when the relevant expression falls at the end of the sentence).
Since the criterion for distinguishing linguistic expressions is based on ontological differences among their significata, Burley's semantic system includes a third kind of expression falling between the simple and complex.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com