RAPGEF1 (Rap guanine nucleotide exchange factor 1) is a guanine nucleotide exchange factor that activates Rap1 and Ras family GTPases through interaction with SH3 domains of adaptor proteins CRK and GRB2 1. The protein functions in multiple signaling pathways including cell adhesion, proliferation, and differentiation 2. RAPGEF1 regulates keratinocyte proliferation and differentiation by promoting cells toward a differentiated state; it is directly targeted by miR-203, which suppresses RAPGEF1 expression during epidermal morphogenesis and keratinocyte differentiation 1. In vascular smooth muscle cells, RAPGEF1 mediates phosphate-induced ERK1/2 phosphorylation through Rap1 activation of the B-Raf/Mek1/2 pathway, affecting vascular phenotype 3. RAPGEF1 is regulated by tyrosine phosphorylation, which can be reversed by TC-PTP phosphatase activity, modulating nerve growth factor-induced neurite outgrowth 2. Genetic variants in RAPGEF1 are associated with type 2 diabetes susceptibility; SNP rs11243444 showed protective effects in Korean populations 4, though associations vary across populations 5. A homozygous RAPGEF1 mutation causes developmental abnormalities including neuropsychiatric disorder, brain and blood vessel defects, demonstrating the protein's essential role in mammalian development 6.