Mutations in the methyl-CpG-binding protein gene MECP2 at Xq28 cause Rett 
syndrome (RTT), an X-linked dominant neurodevelopmental disorder characterized 
by a period of stagnation followed by regression in the development of young 
girls. Mutations were sought in MECP2 in 48 females with classical sporadic RTT, 
seven families with possible familial RTT and five sporadic females with 
features suggestive, but not diagnostic of RTT. Long distance PCR coupled with 
long-read direct sequencing was employed to sequence the entire MECP2 gene 
coding region in all cases. Mutations were identified in 44/55 (80%) unrelated 
classical sporadic and familial RTT patients, but only 1/5 (20%) sporadic cases 
with suggestive but non-diagnostic features of RTT. Twenty-one different 
mutations were identified (12 missense, four nonsense and five frame-shift 
mutations); 14 of these were novel. All missense mutations were located either 
in the methyl-CpG-binding domain or in the transcription repression domain. Nine 
recurrent mutations were characterized in a total of 33 unrelated cases (73% of 
all cases with MECP2 mutations). Significantly milder disease was noted in 
patients carrying missense mutations as compared with those with truncating 
mutations ( P = 0. 0023), and milder disease was associated with late as 
compared with early truncating mutations ( P = 0.0190).