Retinoblastoma (RB) is a neoplasm of retinal origin caused by mutations in RB1, the retinoblastoma tumor suppressor gene. To facilitate genetics counseling and patient management, we adopted a multistep molecular screening assay for detecting RB1 mutations. This assay included DNA sequencing to identify mutations within coding exons and immediate flanking intronic regions, Southern blot analysis to characterize genomic rearrangements, and transcript analysis to characterize potential splicing mutations buried within introns. In a pilot investigation of 180 patients from North America, we identified germline RB1 mutations in 77 out of 85 bilateral RB patients (91%), 7 out of 10 familial unilateral (70%), and 6 out of 85 unilateral patients with no family history of RB (7%). Mutations included 36 novel alterations spanning the entire RB1 gene. Seven of these novel changes were missense or silent mutations. Sequence analysis predicted that, in five out of seven cases, the changes can cause aberrant splicing. This was confirmed by transcript analysis in four out of five cases. In addition, four intronic point mutations within nonconsensus sites activated cryptic splice sites. Without the transcript analysis, the significance of these 11 mutations would have remained undefined. In a separate investigation of a subset of unilateral RB tumors, we identified somatic biallelic RB1 gene inactivation in 34 out of 56 cases (61%) cases. In 14 tumors, only one of the two RB1 mutations could be detected, and in eight tumors, no mutations were detected. The absence of detectable RB1 mutations in eight bilateral cases and eight unilateral tumors suggests that alternative genetic mechanisms may underlie the development of RB in certain individuals.