The expression of a is correct and usable in written English.
It is used as a way to describe something: The expression of a deep sadness was seen in her eyes.
Nor is it the expression of a mental illness.
"The Expression of a nation's subconscious is its secret service".
By then he wore the expression of a haunted man.
It is part of the expression of a performer.
Dancers move through space in a way that echoes the expression of a numerical equation.
He took on the expression of a man who had just bit into a lemon.
As usual, he wears the expression of a man troubled by something.
Genes and environment always interact, and the expression of a particular gene depends on environmental influences.
Its conflicting moral positions can be read as the expression of a painfully divided self.
"Design," said Bailey backstage afterwards, "is the expression of a point of view".
Instead, her apartment is fetishistically devoted to the expression of a strangely personal iconography.
