Facial features- upward slant of eyes, epicanthic folds, low set simple ears, flat occiput, 3rd fontanelle, short neck nuchal fold, single plantar crease, wide gap between 1st and 2nd toe- hypotonia. Prognosis depends karyotype and association of congenital heart disease. Mosaicism occurs when one line of the cell is normal and the other abnormal. Only a percentage of the cells will have abnormal chromosomes. 2% of trisomy21 are mosaics and often have a higher intelligence