[6] Its molecular weight is 70 kDa, and it is a member of the protein-tyrosine kinase family and is a close homolog of SYK.

[7] The importance of ZAP-70 in T cell activation was determined when comparing ZAP-70 expression in patients with SCID (severe combined immunodeficiency).

ZAP-70 is a critical cytoplasmic tyrosine kinase that initiates a signal pathway downstream of an activated T cell receptor.

Lck is moved near the CD3 complex and phosphorylates the tyrosines in the immunoreceptor tyrosine-based activation motifs (ITAMS), creating a docking site for ZAP-70.

[5] In systemic lupus erythematosus, the Zap-70 receptor pathway is missing and the homolog Syk takes its place.

[14] These effects suggest that a deficiency in ZAP-70 results in decreased rates of T cell activation and subsequent signal transductions.