Chromosome 4, Trisomy 4p is a rare chromosomal disorder in which all or a portion of the short arm (p) of chromosome 4 appears three times (trisomy) rather than twice in cells of the body. Associated symptoms and physical findings may vary greatly in range and severity from case to case. Such variability may depend upon the specific length and location of the duplicated (trisomic) portion of chromosome 4p as well as other factors. However, many affected infants may have feeding and breathing difficulties, characteristic malformations of the head and facial (craniofacial) area, and abnormalities of the hands and feet. Additional features may include other skeletal defects, genital abnormalities in affected males, or heart (cardiac) defects. Trisomy 4p is also characterized by severe mental retardation.