text: Testican is a type of proteoglycan. Testican-1 is a highly conserved, multidomain proteoglycan that is most prominently expressed in the thalamus, and is upregulated in activated astroglial cells of the cerebrum. Several functions of this gene product have now been demonstrated in vitro including membrane-type matrix metalloproteinase inhibition, cathepsin L inhibition, and low-affinity calcium binding. The purified gene product has been shown to inhibit cell attachment and neurite extensions in culture. Functions of testican in vivo have yet to be demonstrated in knockout mice or other models. Testican has been shown to carry substantial amounts of chondroitin sulfate as well as other oligosaccharides, but the biological significance of these embellishments is not yet known.
  In humans there are three testicans:
  * SPOCK1 (Testican 1)
  * SPOCK2 (Testican 2)
  * SPOCK3 (Testican 3)
  Testican-1 plays a role in lapatinib resistance, which is a drug used to treat HER2-positive gastric cancer. When testican-1 levels are artificially reduced, sensitivity towards lapatinib was once again increased. This shows the potential for future use in combating drug resistance.