Complex sentence: "Probable validation" has widespread agreement in the medical or scientific community as to its efficacy, whereas "known validation" requires a scientific framework or body of evidence that appears to elucidate the marker's efficacy.
Split into simpler sentences: Probable validation requires widespread agreement in the medical or scientific community as to its efficacy. "Known validation" requires a scientific framework or body of evidence that appears to elucidate the marker's efficacy.