The radial spoke of each group of radial spokes, whether grouped as triplets or doublets, that is most proximal to the base of the cilium. Radial spoke 1 (RS1), similarly to radial spoke 2, is comprised of four domains: 1) a very short base anchored to the A microtubule, 2) an elongaged stalk, 3) a bifurcated neck, and 4) an orthogonal head. The base of RS1 is connected to the tail of the inner dynein arm a/d.
RS1