Probable component of an adaptor protein complex. Adaptor protein complexes are vesicle coat components involved both in vesicle formation and cargo selection. They control the vesicular transport of proteins in different trafficking pathways. May be part of the adaptor protein complex 4 (AP-4), a heterotetramer composed of two large adaptins (epsilon-type subunitand beta-type subunit), a medium adaptin (mu-type subunit) and a small adaptin (sigma-type). Poorly expressed in vegetative cells. Well detected in migrating slugs and highly induced at the finger stage, reaching a peak level at 16 hours of development, and then decreasing to a very low level at 20 hours. Belongs to the adaptor complexes medium subunit family.