1. A method for determining the likelihood that a human subject has severe myoclonic epilepsy of infancy (SMEI), or that the subject is at risk of developing SMEI, comprising, a) analyzing a nucleic acid sample obtained from a subject suspected of having SMEI or being at risk of developing SMEI; b) detecting the presence of one or more SMEI associated mutations in the SCN1A gene in the nucleic acid sample obtained from the subject, wherein the SMEI associated mutations are nucleotide content in SEQ ID NO: 125 selected from the group consisting of: a T nucleotide at position 301; an A nucleotide at position 512; a G nucleotide at position 596; a T nucleotide at position 677; an A nucleotide at position 715; an A nucleotide at position 2837; a C nucleotide at position 3714; a G nucleotide at position 4186; a C nucleotide at position 4321; a G nucleotide at position 4633; an A nucleotide at position 4934; a G nucleotide at position 5119; an A nucleotide at position 5347; deletion of a T nucleotide at position 41; insertion of GTGAATC at position 496; deletion of a C nucleotide at position 1687; deletion of an A nucleotide at position 3231; deletion of nucleotides AA at positions 3561-3562; deletion of a T nucleotide at position 4062; deletion of an A nucleotide at position 4526; a T nucleotide at position 3022; and a T nucleotide at position 4279; an A nucleotide at position 134 of SEQ ID NO: 24; and an A nucleotide at position 88 of SEQ ID NO: 25; and c) determining that the subject has, or is at risk of developing, SMEI when one or more SMEI associated mutations in the SCN1A gene is detected.