Functions redundantly with Npc2b in regulating sterol homeostasis and ecdysteroid biosynthesis, probably by controlling the availability of sterol substrate. Broadly expressed with a higher level of expression in many tissues, including midgut, salivary gland and ventral nerve cord. Expressed both maternally and zygotically. Abnormal sterol distribution in many cells, also lower than normal ecdysteroid levels. Npc2a and Npc2b double mutants undergo apoptotic neurodegeneration. Belongs to the NPC2 family.