*3.1.1 BTK expression in X-linked agammaglobulinemia*

X-linked agammaglobulinemia or Bruton tyrosine kinase (BTK) deficiency is a disorder caused for mutations in the gene encoding BTK. It is a tyrosine-protein kinase localized intracellularly next to the B-cell receptor (BCR), and it is crucial for the B cell development. The expression of BTK protein is evaluated in monocytes or platelets as patients with suspected X-linked agammaglobulinemia have an absent or significantly reduced number of circulating B lymphocytes. This leads to the fact that intracellular BTK protein has to be studied by gating monocytes [25].

See the intracellular staining protocol in Section 3.1.
