Hydroxyproline (HOP) assay was used to quantify collagen content as a measure of fibrosis. Age groups were as follows: cMyBP-C(tWT)—young, 5 to 6 months; old, 19 to 20 months; cMyBP-C(t3SA)—young, 6 months; old, 19 to 20 months; and cMyBP-C(t3SD)—young, 4 months; old, 22 months. HOP levels were similar among all 3 mouse models in both the young and old age groups. However, aging increased HOP levels significantly only in the cMyBP-C(t3SA) hearts. Two-way analysis of variance was used to detect differences between groups. Data are presented as dot plots with mean ±SD.