A genetically modified rodent comprising in its germline (a) a deletion of a nucleotide sequence encoding a C H 1 domain of at least one endogenous immunoglobulin heavy chain constant region gene at an endogenous immunoglobulin heavy chain locus, wherein the at least one endogenous immunoglobulin heavy chain constant region gene is IgG, IgA, IgE, IgD, or a combination thereof, and (b) an immunoglobulin light chain locus that comprises a single rearranged immunoglobulin light chain variable region V L /J L gene sequence comprising V L and J L gene segment sequences, wherein the single rearranged immunoglobulin light chain variable region V L /J L gene sequence is operably linked to an immunoglobulin light chain constant region gene sequence, and optionally, (c) a nucleic acid sequence comprising at least one unrearranged immunoglobulin light chain variable region (V L ) gene segment and at least one unrearranged immunoglobulin light chain joining (J L ) gene segment, wherein the unrearranged V L and J L gene segments are capable of recombining to form a rearranged immunoglobulin light chain variable region (V L /J L ) nucleotide sequence operably linked to the immunoglobulin heavy chain constant region gene comprising the deletion of the nucleotide sequence encoding the C H 1 domain, wherein the rodent further comprises in its serum an antigen-specific single domain antigen binding protein that lacks a functional C H 1 domain, optionally wherein the single domain antigen binding protein is a V L -single domain binding protein encoded by the rearranged immunoglobulin light chain variable region (V L /J L ) nucleotide sequence operably linked to the immunoglobulin heavy chain constant region gene comprising the deletion of the nucleotide sequence encoding the C H 1 domain, wherein the rodent further comprises an IgM heavy chain comprising a functional C H 1 domain, and wherein the IgM heavy chain is associated with a cognate universal light chain encoded by the single rearranged immunoglobulin light chain variable region V L /J L gene sequence comprising V L and J L gene segment sequences operably linked to an immunoglobulin light chain constant region gene sequence. (a) a deletion of a nucleotide sequence encoding a C H 1 domain of at least one endogenous immunoglobulin heavy chain constant region gene at an endogenous immunoglobulin heavy chain locus, wherein the at least one endogenous immunoglobulin heavy chain constant region gene is IgG, IgA, IgE, IgD, or a combination thereof, and (b) an immunoglobulin light chain locus that comprises a single rearranged immunoglobulin light chain variable region V L /J L gene sequence comprising V L and J L gene segment sequences, wherein the single rearranged immunoglobulin light chain variable region V L /J L gene sequence is operably linked to an immunoglobulin light chain constant region gene sequence, and optionally, (c) a nucleic acid sequence comprising at least one unrearranged immunoglobulin light chain variable region (V L ) gene segment and at least one unrearranged immunoglobulin light chain joining (J L ) gene segment, wherein the unrearranged V L and J L gene segments are capable of recombining to form a rearranged immunoglobulin light chain variable region (V L /J L ) nucleotide sequence operably linked to the immunoglobulin heavy chain constant region gene comprising the deletion of the nucleotide sequence encoding the C H 1 domain, <EOS>