The genomic sequence of mouse distal chromosome 2 encodes extended XL and Alex domains, XXL and AlexX, that are potentially much longer than suspected. Analysis of the mouse genomic DNA sequence (GenBank accession no. NT_039211, region 3844806-3847406), which encodes the XL and Alex domains of the Gnas1 complex locus, shows the existence of ORFs of 785.3 and 725 codons that run in parallel and encode in their 3′ end the murine XL and Alex domains of the 422.3-codon XL and 391-codon Alex domains reported in GenBank's AF116268 record. Shown are the nucleotide sequence and, above, the amino acids encoded in the two ORFs. Nucleotides in uppercase are from AF116268 (cDNA), as well as from NT_039211 (genomic sequence); nucleotides in lowercase are from NT_039211 only; nucleotides in italics are from intronic sequences. Amino acids in uppercase indicate the XL domain and its potential N-terminal extension, referred to as XXL; amino acids in lowercase indicate Alex and its potential N-terminal extension, referred to as AlexX (Fig. 4). Also shown, in bold and uppercase characters, are the forward (F) and reverse (R) PCR primers XXF, XF, X3F, and X5R (Fig. 4), which were used to test for the existence of the XXL/AlexX domains.