{{Rsnum
|rsid=121908635
|Chromosome=2
|Orientation=minus
|geno1=(A;A)
|geno2=(A;G)
|geno3=(G;G)
|Gene=PER2
|position=238257003
|Assembly=GRCh38
|GenomeBuild=38.1
|dbSNPBuild=141
|Gene_s=PER2
}}[[rs121908635]], also known as Ser662Gly or S662G, is a SNP in the period circadian clock 2 [[PER2]] gene.

This SNP is associated with the [[sleep disorder]] known as Familial Advanced Sleep Phase Syndrome (FASPS), a highly penetrant autosomal dominant trait.{{PMID|11232563}},{{PMID|10470086}}

Affected individuals are extreme "morning larks"; they show advanced sleep-phase syndrome, awakening and going to bed exceptionally early (by ~4 hours) to maintain a normal quantity and quality of sleep.


{{omim
|id=603426
|rsnum=121908635
|variant=0001
}}{{ClinVar
|rsid=121908635
|Reversed=1
|FwdREF=A
|FwdALT=G
|REF=T
|ALT=C
|RSPOS=239165644
|CHROM=2
|dbSNPBuildID=133
|SSR=0
|SAO=1
|VP=0x050060000000000002110100
|GENEINFO=PER2:8864
|GENE_NAME=PER2
|GENE_ID=8864
|WGT=0
|VC=SNV
|CLNALLE=1
|CLNHGVS=NC_000002.11:g.239165644T>C
|CLNSRC=OMIM Allelic Variant
|CLNORIGIN=1
|CLNSRCID=603426.0001
|CLNSIG=5
|CLNCUI=C1858496
|CLNDBN=Advanced sleep phase syndrome, familial
|Disease=Advanced sleep phase syndrome
|CLNACC=RCV000006717.2
|Tags=RV;PM;OTHERKG;LSD;OM
|CLNDSDB=MedGen:OMIM:Orphanet
|CLNDSDBID=C1858496:604348:164736
}}