BTK and Bruton-type agammaglobulinemia: BTK, a member of the TEC kinase family, was initially identified as a non-receptor protein tyrosine kinase that is inactive in patients with the inherited immunodeficiency disease X-linked agammaglobulinemia (XLA) (Hendriks et al., 2014; Pal Singh et al., 2018; Rip et al., 2018).