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The availability of molecular and cytogenetic techniques has improved our ability to make genetic diagnoses during the newborn period. Clues from both prenatal and familial histories in combination with findings on the infant’s physical examination are vital for appropriate test selection. However, even with such clues, the examiner may be left with a broad differential diagnosis to consider and require input from many pediatric subspecialists. Diagnosing hypotonia in the newborn period is an important example of how genetic testing can aid significantly in timely diagnosis for a critically ill newborn and allow for anticipation of medical interventions by the multidisciplinary team caring for the infant.

When an infant’s postural tone (muscle tone maintained against gravity) is decreased compared with normal, the infant often is described as hypotonic. Phasic hypotonia, a decrease in rapid contraction in response to stretch, also may be...

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