Please, explain meaning of a sentence

I’d like to ask your explanation about punctuation in the sentence below. I’m not sure whether the phrase in parentheses relates to susceptibility factors or it’s separate detail.

Please, clarify this.

Several limitations should be noted. Major limitations are the incomplete sample participation in young adulthood, possible unmeasured underlying susceptibility factors, (e.g., familial influences), and some researchers might question the reliability of the TD instrument for measuring drug dependence.

It is an example of one of the underlying susceptibility factors.

OK. Thanks. I was confused with comma. I wonder why they placed comma after factors and before parentheses. Is it correct?

I presumed it was an error.