Phosphorylase b kinase is involved in the activation of glycogen phosphorylase and is thus involved in the breakdown of glycogen. The enzyme exists as several tissue specific isoenzymes of which the muscle enzyme (rabbit) has been most characterized. It is a multimeric protein composed of four subunits, alpha, beta, gamma and delta. The four subunits are coded on different chromosomes, the alpha, beta and gamma subunit genes being on the X, 16 and 17 chromosomes respectively. The delta subunit is a calmodulin and confers calcium sensitivity on phosphorylase b kinase. Tissue specificity of the enzyme is conferred, at least in some cases, by variation in the gamma subunit. Seven different clinical types of phosphorylase b kinase deficiency have been described. The most common type is X-linked and affects the liver only; other types affect liver, muscle and liver, muscle or heart and have an autosomal recessive mode of inheritance, while in some types the mode of inheritance is not clear. Diagnosis based on the study of erythrocytes or leukocytes can be misleading due to the tissue specific nature of the enzyme, and liver or muscle biopsies may be required.