Complex sentence: Though no accurate estimate of the condition's prevalence has been made, it is understood to be rare: around 150 individuals in the world have been diagnosed with Roberts syndrome, and many of these individuals came from a few affected families.
Split into simpler sentences: Though no accurate estimate of the condition's prevalence has been made, it is understood to be rare. Around 150 individuals in the world have been diagnosed with Roberts syndrome, and many of these individuals came from a few affected families.