RB1 and retinoblastoma: In 136 participants with retinoblastoma tested by the MSK-ACCESS (Memorial Sloan Kettering–Analysis of Circulating cfDNA to Examine Somatic Status) assay, 20 (14.7%) had RB1 mosaicism at variant allele fractions as low at 1%; unlike somatic variants, mosaic RB1 variants persisted in cfDNA after treatment even in the absence of clinical disease.