Sharrard M 1 , Koodiyedath B 1 , Bowen J 1 , Beauchamp N 1 , Johnson D 1 1 Sheffield Children's NHS Trust, Sheffield, United Kingdom GSD IX is historically considered to be a recessively inherited disorder of glycogenolysis caused by mutation in the PHKA2 (X-linked) or PHKB or PHKG2 (autosomal) genes, coding the alpha, beta and gamma subunits of hepatic PhK respectively. PhK is a decahexameric enzyme composed of four of each of the subunits: alpha, beta, gamma, delta. A 19 month old boy with recurrent ketotic hypoglycemia had deficient erythrocyte PhK-0.75micromol/ml/gmHb (10-90) and normal glycogen phosphroylase and debrancher. He was a simple heterozygote for the novel missense mutation of PHKB (p.[=]+[Tyr167Cys]) with no mutation in PHKA2 or PHKG2. Both PHKB allelles generated stable mRNA transcripts. The mother and the half-brother (different father) experienced symptomatic hypoglycemia and PhK deficiency (3.8micromol/ml/gmHb, and undetectable respectively). Both were simple heterozygotes for the PHKB (p.[=]+[Tyr167Cys]) mutation, and both generated two stable mRNA transcripts. In this case the stable mutated betasubunit disrupts enzyme activity. Each PhK enzyme complex contains four beta subunits. If any one of these four is mutated, the resultant enzyme complex has no activity and a total PHK activity is 1/16 of normal. The Tyr167Cys mutation causes autosomal dominant negative inheritance of GSD IX, not previously reported in GSDs, with significant genetic counselling implication. The diagnosis of GSDs has traditionally relied on clinical features combined with biochemical profiling and enzymatic analysis. Diagnosis may then be confirmed by gene sequencing. Disadvantages of this approach include overlapping clinical and biochemical features; cost and time of sequential investigations; the need in some subtypes for tissue biopsy; the scarcity of laboratories offering enzyme analysis and the inherent enzyme instability. Unsupported by sequencing this can easily lead to misdiagnosis with inaccurate prognostic and recurrence risks provided and incorrectly targeted clinical surveillance. We present a new screening process of in-solution DNA capture and NGS to simultaneously screen 18 genes known to cause GSD. 120 bp RNA probes used to create a GSD library covering over 1 Mb Validation studies detected all point mutations, sequence variations and copy number variants. The results from over 30 suspected patients, confirm this technique improves diagnostic accuracy compared to stand alone biochemistry. Improving the original GSD library has allowed multiplexing of samples to further reduce costs. We propose using this technique early in the diagnostic pathway offers a more cost effective, efficient and accurate service and extends accurate diagnosis to patients from around the world who do not have access to the current diagnostic techniques. Glycogen Storage Disease type IV (GSD IV) is caused by deficiency of the glycogen branching enzyme (GBE). The diagnostic feature of the disease is the accumulation of a poorly branched form of glycogen known as polyglucosan. The disease is clinically heterogeneous, with variable tissue involvement and age of disease onset. Complete loss of enzyme activity is lethal in utero or in infancy, affecting primarily muscle and liver. However, enzyme activity as low as 5 to 20% leads to juvenile or adult onset of the disease that affects the central and peripheral nervous system and muscles. By using homologous recombination, two mouse models of GSD IV that reflect this spectrum of enzyme activity and disease were generated. By completely eliminating GBE activity, a phenotype similar to early onset GSD IV with neonatal lethality was created. In contrast, adult animals with some residual GBE activity accumulate significant polyglucosan in virtually all tissues, including the central and peripheral nervous system, and exhibit progressive neuromuscular dysfunction. Unlike in muscle, polyglucosan in liver is a degradable source of glucose and is readily depleted by fasting, emphasizing that there are structural and regulatory differences in glycogen metabolism between these tissues.