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). Enhances the incorporation of serine into phosphatidylserine and sphingolipids. May play a role in providing serine molecules for the formation of myelin glycosphingolipids in oligodendrocytes (By similarity). {ECO:0000250|UniProtKB:Q63175, ECO:0000269|PubMed:26416733, ECO:0000269|PubMed:26416734, ECO:0000269|PubMed:37474505}. This is the function of SERINC5 (serine incorporator 5, Ensembl gene identifier ENSG00000164300).