Transcriptional coactivator involved in neuroepithelial stem cell differentiation and neurogenesis. Involved in particular in lens epithelial cell gene regulation and stress responses. May play an important role in lens epithelial to fiber cell terminal differentiation. May play a protective role during stress-induced apoptosis. Isoform 2 is a more general and stronger transcriptional coactivator. Isoform 2 may also act as an adapter to coordinate pre-mRNA splicing. Cellular cofactor for lentiviral integration. Monomer (PubMed:15895093). Interacts with IFRD1/PC4 (PubMed:9822615). Isoform 2 interacts with SFRS1 (PubMed:9885563). Isoform 1 interacts (via IBD domain) with POGZ (via IBM motif) and CDCA7L (via IBM motifs) (PubMed:19244240, PubMed:25082813, PubMed:29997176). Interacts (via IBD domain) with KMT2A (via IBM motifs) with a moderate affinity whereas interacts with the KMT2A-MEN1 complex with a greater affinity; MEN1 enhances interaction of KMT2A with PSIP1 (PubMed:22327296, PubMed:25305204, PubMed:25082813, PubMed:29997176). Interacts with fusion protein KMT2A-MLLT3 (PubMed:25305204). Interacts (via IBD domain) with IWS1 (via IBM motif), MED1 (via IBM motif) and DBF4 (via IBM motifs) (PubMed:29997176). (Microbial infection) Interacts (via IBD domain) with human HIV-1 integrase protein (HIV-1 IN), determining its nuclear localization, its tight association with chromatin and its protection from the proteasome. (Microbial infection) Interacts with HIV-2 IN. Remains chromatin-associated throughout the cell cycle. Widely expressed. Expressed at high level in the thymus. Expressed in fetal and adult brain. Expressed in neurons, but not astrocytes. Markedly elevated in fetal as compared to adult brain. In the adult brain, expressed in the subventricular zone (SVZ), in hippocampus, and undetectable elsewhere. In the fetal brain, expressed in the germinal neuroepithelium and cortical plate regions. Citrullinated by PADI4. A chromosomal aberration involving PSIP1 is associated with pediatric acute myeloid leukemia (AML) with intermediate characteristics between M2-M3 French-American-British (FAB) subtypes. Translocation t(9;11)(p22;p15) with NUP98. The chimeric transcript is an in-frame fusion of NUP98 exon 8 to PSIP1 exon 4. Less active than isoform 2 as transcriptional coactivator, but more abundant in cells. Belongs to the HDGF family.