Different presentations of CEACAM crystal structures. (A–C) Published CEACAM PDB data files are presented as ribbon models (top), space-filling models with atomic resolution (middle), and nuclear scattering models at a 20-Å resolution (bottom). The right image of each structure is rotated 90° around the x axis with respect to the corresponding left image. (A) Ectodomain of a mouse CEACAM1 isoform with the D1 domain (top right) fused to the D4 domain (bottom left; PDB accession no. 1L6Z). (B) Two D1 domains of human CEACAM5 (PDB accession no. 2QSQ) in the asymmetric unit (asym unit) in contact via their C″C′CFG faces. (C) Two D1 domains of human CEACAM1 (PDB accession no. 2GK2) in the asymmetric unit in contact via their ABED faces. Bars, 2.5 nm. (D) Ribbon model of the human CEACAM1 D1 domain shown at three different angles to visualize the C″C′CFG and the ABED β sheets.