Table 5.

A summary of the severity classification of the variants described by Curtis et al. [161].

Class	ABCA4 Expression	Basal ATPase Activity	Stimulation by N-Ret-PE
1	Significantly reduced	<50%	Not stimulated
2	Partial reduction	50–80%	Modestly stimulated
3	Comparable to WT	Comparable to WT	Comparable to WT