Yes, quarks are considered to be the fundamental particles that make up protons and neutrons, which are the building blocks of atomic nuclei. Quarks are elementary particles and are currently believed to be the smallest known particles. They are not directly observable and are always found bound together in groups to form composite particles called hadrons. Protons and neutrons, for example, are composed of three quarks each. However, it's important to note that our understanding of the fundamental nature of particles is based on the current knowledge in the field of particle physics, and further discoveries or developments may refine or expand this understanding in the future.