The cilary pro-basal body is an immature, partially assembled form of a ciliary basal body found next to the basal body of a cilium. Pro-basal bodies are not capable of nucleating a cilium. As the cell progresses through the cell cycle, continuing assembly will convert the pro-basal body into a mature basal body that is capable of nucleating a cilium.
Pro-basal bodies are distinct from basal bodies as they are not at the base of a cilium and are not capable of nucleating a cilium. While immature, they contain some structures in common with the mature basal body and also may contain proteins unique to the immature state. Note that cilia and eukaryotic flagella are deemed to be equivalent. In many eukaryotic cells, 'ciliary basal body' (GO:0036064) and 'centriole' (GO:0005814) represent a common entity that cycles through its function in cell division, then ciliogenesis, then cell division again. However, these structures are modified extensively as they transition into each other, and may contain different proteins, specific to each component. In other eukaryotic cells, centrioles are not involved in cell division but only in cilium assembly, which is thought to be the ancestral role of the centriole/basal body.