Polypeptide comprising a first and a second immunoglobulin single variable domain, wherein - said first ISV binds to CD4 present on the surface of a cell; - said second ISV binds to CXCR4 present on the surface of said cell; and wherein said first ISV consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which said first ISV is chosen from the group consisting of - CDR1 is SEQ ID NO: 82, CDR2 is SEQ ID NO: 88, and CDR3 is SEQ ID NO: 96; - CDR1 is SEQ ID NO: 83, CDR2 is SEQ ID NO: 89, and CDR3 is SEQ ID NO: 97; - CDR1 is SEQ ID NO: 84, CDR2 is SEQ ID NO: 90, and CDR3 is SEQ ID NO: 98; and - CDR1 is SEQ ID NO: 85, CDR2 is SEQ ID NO: 91, and CDR3 is SEQ ID NO: 99; and, wherein said second ISV consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which said second ISV is chosen from the group consisting of - CDR1 is SEQ ID NO: 34, CDR2 is SEQ ID NO: 48, and CDR3 is SEQ ID NO: 67; - CDR1 is SEQ ID NO: 34, CDR2 is SEQ ID NO: 49, and CDR3 is SEQ ID NO: 68; - CDR1 is SEQ ID NO: 35, CDR2 is SEQ ID NO: 50, and CDR3 is SEQ ID NO: 69; - CDR1 is SEQ ID NO: 36, CDR2 is SEQ ID NO: 51, and CDR3 is SEQ ID NO: 70; - CDR1 is SEQ ID NO: 37, CDR2 is SEQ ID NO: 52, and CDR3 is SEQ ID NO: 71; - CDR1 is SEQ ID NO: 35, CDR2 is SEQ ID NO: 53, and CDR3 is SEQ ID NO: 72; - CDR1 is SEQ ID NO: 38, CDR2 is SEQ ID NO: 54, and CDR3 is SEQ ID NO: 73; - CDR1 is SEQ ID NO: 39, CDR2 is SEQ ID NO: 55, and CDR3 is SEQ ID NO: 74; and - CDR1 is SEQ ID NO: 40, CDR2 is SEQ ID NO: 56, and CDR3 is SEQ ID NO: 75. - said first ISV binds to CD4 present on the surface of a cell; - said second ISV binds to CXCR4 present on the surface of said cell; and wherein said first ISV consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which said first ISV is chosen from the group consisting of - CDR1 is SEQ ID NO: 82, CDR2 is SEQ ID NO: 88, and CDR3 is SEQ ID NO: 96; - CDR1 is SEQ ID NO: 83, CDR2 is SEQ ID NO: 89, and CDR3 is SEQ ID NO: 97; - CDR1 is SEQ ID NO: 84, CDR2 is SEQ ID NO: 90, and CDR3 is SEQ ID NO: 98; and - CDR1 is SEQ ID NO: 85, CDR2 is SEQ ID NO: 91, and CDR3 is SEQ ID NO: 99; - CDR1 is SEQ ID NO: 82, CDR2 is SEQ ID NO: 88, and CDR3 is SEQ ID NO: 96; - CDR1 is SEQ ID NO: 83, CDR2 is SEQ ID NO: 89, and CDR3 is SEQ ID NO: 97; - CDR1 is SEQ ID NO: 84, CDR2 is SEQ ID NO: 90, and CDR3 is SEQ ID NO: 98; and - CDR1 is SEQ ID NO: 85, CDR2 is SEQ ID NO: 91, and CDR3 is SEQ ID NO: 99; and, wherein said second ISV consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which said second ISV is chosen from the group consisting of - CDR1 is SEQ ID NO: 34, CDR2 is SEQ ID NO: 48, and CDR3 is SEQ ID NO: 67; - CDR1 is SEQ ID NO: 34, CDR2 is SEQ ID NO: 49, and CDR3 is SEQ ID NO: 68; - CDR1 is SEQ ID NO: 35, CDR2 is SEQ ID NO: 50, and CDR3 is SEQ ID NO: 69; - CDR1 is SEQ ID NO: 36, CDR2 is SEQ ID NO: 51, and CDR3 is SEQ ID NO: 70; - CDR1 is SEQ ID NO: 37, CDR2 is SEQ ID NO: 52, and CDR3 is SEQ ID NO: 71; - CDR1 is SEQ ID NO: 35, CDR2 is SEQ ID NO: 53, and CDR3 is SEQ ID NO: 72; - CDR1 is SEQ ID NO: 38, CDR2 is SEQ ID NO: 54, and CDR3 is SEQ ID NO: 73; - CDR1 is SEQ ID NO: 39, CDR2 is SEQ ID NO: 55, and CDR3 is SEQ ID NO: 74; and - CDR1 is SEQ ID NO: 40, CDR2 is SEQ ID NO: 56, and CDR3 is SEQ ID NO: 75. - CDR1 is SEQ ID NO: 34, CDR2 is SEQ ID NO: 48, and CDR3 is SEQ ID NO: 67; - CDR1 is SEQ ID NO: 34, CDR2 is SEQ ID NO: 49, and CDR3 is SEQ ID NO: 68; - CDR1 is SEQ ID NO: 35, CDR2 is SEQ ID NO: 50, and CDR3 is SEQ ID NO: 69; - CDR1 is SEQ ID NO: 36, CDR2 is SEQ ID NO: 51, and CDR3 is SEQ ID NO: 70; - CDR1 is SEQ ID NO: 37, CDR2 is SEQ ID NO: 52, and CDR3 is SEQ ID NO: 71; - CDR1 is SEQ ID NO: 35, CDR2 is SEQ ID NO: 53, and CDR3 is SEQ ID NO: 72; - CDR1 is SEQ ID NO: 38, CDR2 is SEQ ID NO: 54, and CDR3 is SEQ ID NO: 73; - CDR1 is SEQ ID NO: 39, CDR2 is SEQ ID NO: 55, and CDR3 is SEQ ID NO: 74; and - CDR1 is SEQ ID NO: 40, CDR2 is SEQ ID NO: 56, and CDR3 is SEQ ID NO: 75.