The function of PPID (peptidylprolyl isomerase D, ENSG00000171497) is as follows. PPIase that catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and may therefore assist protein folding (PubMed:11350175, PubMed:20676357). Proposed to act as a co- chaperone in HSP90 complexes such as in unligated steroid receptors heterocomplexes. Different co-chaperones seem to compete for association with HSP90 thus establishing distinct HSP90-co-chaperone- receptor complexes with the potential to exert tissue-specific receptor activity control. May have a preference for estrogen receptor complexes and is not found in glucocorticoid receptor complexes. May be involved in cytoplasmic dynein-dependent movement of the receptor from the cytoplasm to the nucleus. May regulate MYB by inhibiting its DNA- binding activity. Involved in regulation of AHR signaling by promoting the formation of the AHR:ARNT dimer; the function is independent of HSP90 but requires the chaperone activity. Involved in regulation of UV radiation-induced apoptosis. Promotes cell viability in anaplastic lymphoma kinase-positive anaplastic large-cell lymphoma (ALK+ ALCL) cell lines. {ECO:0000269|PubMed:11350175, ECO:0000269|PubMed:18708059, ECO:0000269|PubMed:20676357, ECO:0000269|PubMed:22681779, ECO:0000269|PubMed:23220213, ECO:0000269|PubMed:9659917}. (Microbial infection) May be involved in hepatitis C virus (HCV) replication and release. {ECO:0000269|PubMed:19932913, ECO:0000269|PubMed:21711559}.