The first mutation associated with ADNFLE is a serine to phenylalanine transition at position 248 (S248F), located in the second transmembrane spanning region of the gene encoding a nicotinic acetylcholine receptor α4 subunit. Using the numbering based on the human "CHRNA4" protein, this mutation is called S280F. Receptors containing this mutant subunit are functional, but desensitize at a much faster pace compared to wild-type only receptors. These mutant containing receptors also recover from desensitization at a much slower rate than wild-type only receptors. These mutant receptors also have a decreased single channel conductance than wild-type and have a lower affinity for acetylcholine. Also importantly, this mutation along with the others in "CHRNA4" produce receptors less sensitive to calcium.