A genetics evaluation was requested for a 6-week-old infant with multiple congenital malformations including mild craniofacial anomalies, truncal hypotonia, hypospadias, and a ventriculoseptal defect. Blood obtained for chromosome analysis revealed an abnormal chromosome 4. Paternal chromosome analysis showed a 46,XY, inv ins (3;4)(p21.32; q25q21.21, inv(4)(p15.3q21.2) karyotype. Therefore, the proband's chromosome 4 was the unbalanced product of this insertional translocation from the father resulting in partial monosomy 4q. Additionally, the derivative 4 had a pericentric inversion which was also seen in the father's chromosome 4. During genetic counseling, the proband's 2-year-old brother was evaluated. He was not felt to be abnormal in appearance, but was described as having impulsive behavior. Chromosome analysis on this child revealed 46, XY,der(3)inv ins(3;4)(~21.32;q25q21.2)pat. This karyotype results in partial trisomy 4q. FISH using two-color "painting" probes for chromosomes 3 and 4 confirmed the G-banded interpretation in this family. The segregation seen in this family resulted in both reciprocal products being observed in the two children, with partial 4q monosomy showing multiple congenital anomalies, and partial 4q trisomy showing very few phenotypic abnormalities.