MyBP family schematic. (A) The five members of the MyBP family include MyBP-H, MyBP-HL, and cardiac, slow skeletal, and fast skeletal MyBP-C. All MyBP family members have an Ig-Fn-Ig C′-terminal region required to bind to the thick filament (horizontal line). All MyBP family members also have an unstructured, proline/alanine-rich domain near or at the N′-terminal end of the protein (gradient circle). The sequences of these unstructured domains have low similarity between MyBP family members. (B) Schematic of a half-sarcomere showing the MyBP binding repeats, each separated by 43 nm, that defines the C-zone. (C) Myosin thick filament regulation schematics with different occupancies of MyBP-C and/or MyBP-H. While MyBP-H may fill in MyBP sites to remove MyBP-C regulation, MyBP-H has now been shown to have its own intrinsic function, as the C-zone portion of the thick filament regulated with only MyBP-H shows slowed actin sliding velocities compared with regions outside the C-zone with no MyBP regulation.