29 CFR Part 1630, Regulations to Implement the Equal Employment Provisions of the ADA (Title I Regulations with amendments issued through May 2016)

(2) A job function may be considered essential for any of several reasons, including but not limited to the following:

(i) The function may be essential because the reason the position exists is to perform that function;

(ii) The function may be essential because of the limited number of employees available among whom the performance of that job function can be distributed; and/or

(iii) The function may be highly specialized so that the incumbent in the position is hired for his or her expertise or ability to perform the particular function.