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Probability of a cryptic (undetected) heritable RB1 pathogenic variant in isolated case of unilateral retinoblastoma

Lymphocyte DNA Tumour DNA Probability of cryptic heritable RB1 pathogenic variant
  • Not tested
  • 1.3%
  • No RB1 pathogenic variant
  • No RB1 pathogenic variant detected
  • 11.6%
  • No RB1 pathogenic variant detected
  • 1 RB1 pathogenic variant detected
  • 6.2%
  • No RB1 pathogenic variant detected
  • 2 RB1 pathogenic variant detected
  • small risk (<1%) of low level mosaicism in lymphocytes which may involve the germline


  1. Sanger sequencing and MLPA is used*
  2. a germline RB1 pathogenic variant is present in 12% of unilateral, non-familial RB; 
  3. the laboratory can detect 90% of germline and somatic RB1 pathogenic variants;
  4. the laboratory is able to detect mosaicism above 20%; 
  5. the identification of a pathogenic variant on one allele is independent of the presence of a pathogenic variant on the other i.e. in tumour tissue from a child with heritable RB, the probability of detecting both pathogenic variants is 0.81 (p2), one pathogenic variant 0.18 (2pq) and no pathogenic variants 0.01 (q2).

*level of detected mosaicism may vary depending on technology (ie deep sequencing may detect lower levels of mosaicism) 

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12 Dec 2019