Disclosed is polypeptide heterodimer, comprising: (a) a first single chain polypeptide (SCP-I) comprising from one to four binding domains that specifically bind from one to four targets, a hinge (H-I), an immunoglobulin heterodimerisation domain (HD-I), and an immunoglobulin CH2 and CH3 domain of IgG1, IgG2, IgG3, IgG4, IgA1, IgA2, IgD, or any combination thereof (CH2CH3-I) and (b) a second single chain polypeptide (SCP-II) comprising from zero to four binding domains that specifically bind from zero to four targets, a hinge (H-II), an immunoglobulin heterodimerisation domain (HD-II), and an immunoglobulin CH2 and CH3 domain of IgG1, IgG2, IgG3, IgG4, IgA1, IgA2, IgD, or any combination thereof (CH2CH3-II) wherein (i) the immunoglobulin heterodimerisation domain of the first single chain polypeptide (HD-I) and the immunoglobulin heterodimerisation domain of the second single chain polypeptide (HD-II) preferentially associate with each other to form a polypeptide heterodimer comprised of the first single chain polypeptide (SCP-I) and the second single chain polypeptide (SCP-II), and (1) the immunoglobulin heterodimerisation domain of the first single chain polypeptide (HD-I) comprises a first immunoglobulin CH1 region, and the immunoglobulin heterodimerisation domain of the second single chain polypeptide (HD-II) comprises a first immunoglobulin CL region, or (2) the immunoglobulin heterodimerisation domain of the first single chain polypeptide (HD-I) comprises a first immunoglobulin CL region, and the immunoglobulin heterodimerisation domain of the second single chain polypeptide (HD-II) comprises a first immunoglobulin CH1 region and (ii) wherein the first immunoglobulin CL region is an altered human immunoglobulin C&kgr region with one or more amino acids of a wild type human C&kgr region substituted at N29, N30, Q52, V55, T56, S68, or T70, provided that the polypeptide heterodimer comprises at least two binding domains (BD1 and BD2) that specifically bind at lea
