PROJECT SUMMARY The overall goal of this project is to develop tandem mass spectrometry for newborn screening of a subset of congenital diseases in neonates in which initiation of early treatment leads to a better patient quality of life. A punch from a dried blood spot on a newborn screening card is used as the sample to measure the activities of relevant enzymes and to quantify disease biomarkers. These assays are multiplexed together in order to minimize the workload in the newborn screening laboratory so that an increasing number of diseases can be included (as new treatments become available). Tandem mass spectrometry is also being developed for use in the second stage of newborn screening in order to reduce the number of false positives resulting from only a single stage of analysis. This is important to reduce patient follow-up and associated costs and to reduce family anxiety. A third component is to test the hypothesis that the lower the level of residual function of a particular protein, the more severe is the disease. Predicting disease severity including age of onset of symptoms is important in planning follow- up and treatment options in newborns who screen positive for a disease.