The human RB1 gene spans 180 kb of DNA^[39] and contains 27 exons. It encodes a 4.8-kb mRNA.^[40] Its protein product (pRb) is a 110-kDa phosphoprotein composed of 928 amino acids. This protein functions as a regulator at the cell cycle checkpoint between the G1 and S-phase. When inactivated, transcription of downstream genes that promote progression through the cell cycle occurs. Along with p107 and p130, pRb belongs to the pocket protein family.

Retinoblastoma likely arises from either a precursor cone photoreceptor or a multipotent retinoblast. The retinal cells in these patients develop and mature between the third month postconception and the age of 4 years, being susceptible to mutations at any point during this time.