Focal adhesion kinase subfamily consists of the non-receptor proline-rich protein tyrosine kinases (PTKs). Two members of the family are focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (PYK2). These two kinases have molecular mass between 110–125 kDa and are closely related in their structure. The presence of two proline-rich motifs within the C-terminal domains is conserved. Pyk2 is expressed primarily in brain and hematopoietic cells and becomes activated in response to stimulation through numerous receptors, including integrins, chemokine receptors, and antigen receptors. It is also expressed in both in bone-forming osteoblasts and bone-resorbing osteoclasts, and thought to have positive role in osteoblast maturation and bone resorption.Phosphorylation of Pyk2 leads to the recruitment of Src family kinases and the activation of Erks. Pyk2 also interacts with and phosphorylates the focal adhesion-related protein, paxillin, and other cytoskeletal proteins, suggesting its pivotal role in various cellular events.