Over the last 10 years, the animal cap of theXenopus laevisembryo has proved to be a versatile test tissue for a variety of molecules involved not only in animal development but also vertebrate cell regulation in general. These molecules include growth factors (1 –3 ), cell surface receptors (4 –6 ), signal transduction molecules (7 ,8 ), transcription factors (9 ), and extracellular matrix molecules (10 ). The “animal cap assay” provides a simple, quick, inexpensive, and quantitative bioassay for biological activity of both cloned genes and purified or unpurified proteins.
