Mental retardation is defined by significant limitations in intellectual function and adaptive behavior that occur before 18 years of age.Many chromosomal diseases come with mental retardation.We reported two Chinese families with partial trisomy 9p and other chromosome partial monosomy,clinical features of mental retardation and mild facial and pinkie anomalies.In the family 1,we showed that the proband carried a trisomy 9p21.3→pter and monosomy 21q22.3→qter by using fluorescence in situ hybridization analysis.Molecular genetic analysis defined the precise breakpoint on chromosome 9p between markers D9S1846 and D9S171,an interval of about 2.9 Mb on 9p21.3,and the breakpoint on chromosome 21q between markers D21S1897 and D21S1446,a region of about 1.5 Mb on 21q22.3.In the family 2,a patient with trisomy 9p21.3→pter and monosomy 5p15.33→pter,and a de novo maternal balanced translocation between chromosomes 5 and 9 was identified in his mother.Cytogenetic and molecular genetic analysis defined the precise breakpoints on chromosome 9p21.3 and chromosome 5p15.33.Further clinical investigation found that any individual had no refractoriness eczema disease except the proband in this family.These results further implicate that trisomy 9p is associated with mental retardation,and that there may be key gene duplication on chromosome 9p21.3→9pter responsible for mental retardation and mild facial anomaly.This result has been applied successfully in prenatal diagnosis of the second family.