The function of LMBRD1 (LMBR1 domain containing 1, Ensembl gene identifier ENSG00000168216) is as follows. Lysosomal membrane chaperone required to export cobalamin (vitamin B12) from the lysosome to the cytosol, allowing its conversion to cofactors (PubMed:19136951). Targets ABCD4 transporter from the endoplasmic reticulum to the lysosome (PubMed:27456980). Then forms a complex with lysosomal ABCD4 and cytoplasmic MMACHC to transport cobalamin across the lysosomal membrane (PubMed:25535791). Acts as an adapter protein which plays an important role in mediating and regulating the internalization of the insulin receptor (INSR) (By similarity). Involved in clathrin-mediated endocytosis of INSR via its interaction with adapter protein complex 2 (By similarity). Essential for the initiation of gastrulation and early formation of mesoderm structures during embryogenesis (By similarity). {ECO:0000250|UniProtKB:Q8K0B2, ECO:0000269|PubMed:19136951, ECO:0000269|PubMed:27456980, ECO:0000303|PubMed:25535791}. [Isoform 3]: (Microbial infection) May play a role in the assembly of hepatitis delta virus (HDV). .