'Knudson's two-hit' hypothesis is associated with
Crohn's disease
Melanoma
Ulcerative colitis
Retinoblastoma
Harshmohan textbook of pathology 7th edition. 
☆ Familial retinoblastoma comprises 40% of cases and may be bilateral. 
In this cases, all somatic cells inherit one mutant RB gene from a carrier parent. Later during life, the other mutational event of 2nd allele affecting the somatic cells occurs.
This forms the basis of two hot hypothesis given by Knudson in 1971.