The term Src, an abbreviated form of the word sarcoma, denotes a family of tyrosine kinases that are proto-oncogenic. Generally animal cells contain at least one of the Src families of tyrosine kinases, the enzymes that are able to transfer a phosphate group from ATP to a tyrosine residue. The phosphorylation of proteins by tyrosine kinases is crucial for signal transduction in multicellular animals. V-Src is the mutated version of cellular Src (C-Src) that has lost its characteristic ability to inhibit enzyme activity. C-Src was targeted in the human osteosarcoma epithelial cells featured in the digital video sequences presented in this section with mEmerald, while H2B was fluorescently labeled with mApple.