HPS4 and Hermansky-Pudlak syndrome: By contrast, the corresponding HPS model mice, pale ear (Hps1-deficient) and light ear (Hps4-deficient) exhibit pigmentation on their backs comparable to WT mice but develop hypopigmentation in several other parts of their bodies, including their ears, tail, and feet [11], suggesting the existence of a difference between the mechanisms of melanogenesis on the back skin and the ears [37].