This lesion most commonly involves soft tissues of shoulder, axilla, and upper arm in infants (usually within first 2 yrs. of life). Boys are more commonly affected. Grossly, it has infiltrative appearance. It is composed of three distinctive tissue types: 1) well-differentiated fibroblastic/myofibroblastic cells, 2) mature adipose tissue, and 3) whorls of primitive mesenchymal areas.