"Following a period of normal development, with a normal prenatal and perinatal period with normal developmental progress for the first 5–6 months of life, a deceleration of head growth is seen, followed by behavioural regression and arrest of motor and cognitive development (reduction or loss of acquired skills such as purposeful hand function, vocalisation, and communication skills) 1."
"Rett syndrome is a rare neurodegenerative disease seen in young girls in the 2nd to 4th months of life. The majority of cases result from a mutation in the MECP2 gene; a more severe subset of the disease arises from a CDKL5 mutation 1."