San Francisco's premier showcase for modern art is housed in a delightful building (1995, Mario Botta) that seems to gaze benignly out over Yerba Buena Center for the Arts. Together these insitutions form the creative heart of the district. Botta's design is a striking composition of symmetrically arranged, stepped-back, brick-clad masses. Piercing the center of the building is a massive shaft, set off from the brick by stripes of alternating black and white stone. Botta created a huge skylight by angling the top of the cylinder toward 3rd Street and facing the slanted surface with glass. This suffuses the stunning central atrium with natural light.Now numbering more than 20,000 works of art, the painting and sculpture collections represent all major movements of modern art, with significant strengths in early Modernism and works by California and Bay Area artists. Rotating selections from these collections are mounted on the second and fifth floors. The museum also has significant holdings in photography and architectural models and drawings, selections of which are on the third and second floors, respectively. Large temporary exhibits are displayed on the fourth floor.