Restriction factor required to restrict infectivity of lentiviruses, such as HIV-1: acts by inhibiting an early step of viral infection. Impairs the penetration of the viral particle into the cytoplasm (PubMed:26416733, PubMed:26416734). Non-ATP-dependent, non- specific lipid transporter for phosphatidylserine, phosphatidylcholine, and phosphatidylethanolamine. Functions as a scramblase that flips lipids in both directions across the membrane. Phospholipid scrambling results in HIV-1 surface exposure of phosphatidylserine and loss of membrane asymmetry, which leads to changes in HIV-1 Env conformation and loss of infectivity (PubMed:37474505). {ECO:0000269|PubMed:26416733, ECO:0000269|PubMed:26416734, ECO:0000269|PubMed:37474505}. This is the function of Ensembl gene identifier ENSG00000132824 (SERINC3, serine incorporator 3).