Down syndrome, previously called “mongolism,” is a relatively common condition resulting from the presence of an extra chromosome, number 21, in all the cells of the body. In each human cell, there are 23 chromosome pairs containing basic genetic material that organizes the body’s development and physiological functioning. Each pair has a distinctive size and conformation and can be readily identified on microscopic examination. Chromosome pair number 21 is one of the smaller chromosomes. In Down syndrome there are usually three (trisomy) rather than two number 21 chromosomes (trisomy 21: found in 95 to 98 percent of all cases). In a small number of children with Down syndrome, the extra number 21 chromosome is attached to a chromosome of a larger pair (numbers 13 to 15; translocation Down syndrome–about 2 percent of all cases). In some children with the features of Down syndrome, the extra chromosome is present in less than 90 percent of the cells (mosaic Down syndrome–about 2 to 4 percent of all cases). Down syndrome is the most frequently occurring chromosome abnormality in live-born humans, and is also among the most frequently identified chromosomal abnormalities, representing about 4 percent of all aborted fetuses (Lilienfeld 1969). Down syndrome is usually recognizable at birth as a cluster of physical and neurological abnormalities (see Clinical Manifestations), which develop in a characteristic fashion during the life cycle.