Fig. 4. Signal transduction pathways activated by EGF in the lacrimal gland. ErbB1, the EGF receptor; ErbB, the EGF family of growth factor receptors; PLC, phospholipase C; PI3K, phosphatidylinositol 3 kinase; P, phosphorylated tyrosine residue on the EGF receptor; raf, mitogen-activated protein kinase kinase kinase; MEK, mitogen-activated protein kinase kinase; MAPK, p44/p42 mitogen-activated protein kinase; PKC, protein kinase C. (Reprinted from Hodges RR, Dartt DA: Regulatory pathways in lacrimal gland epithelium. Int Rev Cytol 231:129, 2003, with permission.)